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THE  UBRARY  OF  THE 

UNIVERSITY  OF 

NORTH  CAROUNA 

AT  CHAPEL  HILL 


ENDOWED  BY  THE 

DIALECTIC  AND  PHILANTHROPIC 

SOCIETIES 


QG73 
.S8i4 

1881 


UNIVERSITY  OF  N.C.  AT  CHAPEL  HILL 


00031907562 


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THE   miERNATIONAL   SCIEJ^TIFIC   SERIES. 
VOLUME    VII. 


INTERNATIONAL  SCIENTIFIC  SERIES. 


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New  York:   D.  APPLETON  &  CO.,  1,  3,  &  5  Bond  Street. 


THE   INTERNATIONAL   SCIENTIFIC   SERIES. 


THE 


CONSERVATION  OF  ENERGY. 


BT 


BALFOUR  STEWART,  LL.D.,  F.R.S., 

PE0FES30K   OF  NATUEAL   PnlLOSOPHY   AT  TUB   OWENS   COLLEGE,   MANCHESTER. 


WITH  AN  APPENDIX, 


TREATING  OF  TUE  VITAL  AND    MENTAL  APPLICAXJi^^^yOOF'MlB?;-^ 


DOCTEINE 


NEW  YORK : 
D.    APPLETON   AND    COMP 

1,    3,    AND    5    BOND     STREET 
1881. 


Enteked,  according  to  Act  of  Congress,  in  the  year  1ST4,  by 

D.   APPLETON  &  COMPANY, 

In  the  Office  of  tlie  Librarian  of  Congress,  at  Washington. 


NOTE  TO  THE  AMERICAN  EDITION. 


Tub  great  prominence  wliich  the  modern  doctrine 
of  the  Conservation  of  Energy  or  Correlation  of  Forces 
has  lately  assumed  in  the  world  of  thought,  has  made 
a  simple  and  popular  explanation  of  the  subject  very 
desirable.  The  present  work  of  Dr.  Balfour  Stewart, 
contributed  to  the  International  Scientific  Series,  fully 
meets  this  requirement,  as  it  is  probably  the  clearest 
and  most  elementary  statement  of  the  question  that  has 
yet  been  attempted.  Simj)le  in  language,  copious  and 
familiar  in  illustration,  and  remarkably  lucid  in  the 
presentation  of  facts  and  principles,  his  little  treatise 
forms  just  the  introduction  to  the  great  problem  of  the 
interaction  of  natural  forces  that  is  required  by  general 
readers.  But  Professor  Stewart  having  confined  him- 
self mainly  to  the  physical  aspects  of  the  subject,  it  was 
desirable  that  his  idews  should  be  supplemented  by  a 
statement  of  the  operation  of  the  principle  in  the 
spheres  of  life  and  mind.  An  Appendix  has,  accord- 
ingly, been  added  to  the  American  edition  of  Dr.  Stew- 


y[  NOTE  TO  THE  AMEEICAN  EDITION 

art's  work,  in  which  these  applications  of  the  law  are 
considered. 

Professor  Joseph  Le  Conte  published  a  very  able 
essay  fourteen  years  ago  on  the  Correlation  of  the 
Physical  and  Yital  Forces,  which  was  extensively  re- 
printed abroad,  and  placed  the  name  of  the  author 
among  the  leading  interpreters  of  the  subject.  His 
mode  of  presenting  it  was  regarded  as  peculiarly  happy, 
and  was  widely  adopted  by  other  writers.  After  fur- 
ther investigations  and  more  mature  reflection,  he  has 
recently  restated  his  views,  and  has  kindly  furnished 
the  revised  essay  for  insertion  in  this  volume. 

Professor  A.  Bain,  the  celebrated  Psychologist  of 
Aberdeen,  who  has  done  so  much  to  advance  the  study 
of  mind  in  its  physiological  relations,  prepared  an  in- 
teresting lecture  not  long  ago  on  the  "  Correlation  of  the 
Nervous  and  Mental  Forces,"  which  was  read  with  much 
interest  at  the  time  of  its  publication,  and  is  now  re- 
printed as  a  suitable  exposition  of  that  branch  of  the 
subject.  These  two  essays,  by  carrying  out  the  prin- 
ciple in  the  field  of  vital  and  mental  phenomena,  will 
serve  to  give  completeness  and  much  greater  value  to 
the  present  volume. 

New  York,  December,  1873. 


PEE  FAC  E. 


"We  may  regard  tlie  Universe  in  the  light  of  a  vast 
physical  machine,  and  our  knowledge  of  it  may  be 
conveniently  divided  into  two  branches. 

The  one  of  these  embraces  what  we  know  regarding 
the  structure  of  the  machine  itself,  and  the  other  what 
we  know  regarding  its  method  of  working. 

It  has  appeared  to  the  author  that,  in  a  treatise  like 
this,  these  two  branches  of  knowledge  ought  as  much 
as  possible  to  be  studied  together,  and  he  has  therefore 
endeavored  to  adopt  this  course  in  the  following  pages. 
He  has  regarded  a  universe  composed  of  atoms  with 
some  sort  of  medium  between  them  as  the  machine, 
and  the  laws  of  energy  as  the  laws  of  working  of  this 
machine. 


viii  PREFACE. 

The  first  chapter  embraces  what  we  know  regarding 
atoms,  and  gives  also  a  definition  of  Energy.  The  varions 
forces  and  energies  of  iJ^atnre  are  thereafter  enumerated, 
and  the  law  of  Conservation  is  stated.  Then  follow  the 
various  transmutations  of  Energy,  according  to  a  list,  for 
which  the  author  is  indebted  to  Prof.  Tait.  The  fifth 
chapter  gives  a  short  historical  sketch  of  the  subject, 
ending  with  the  law  of  Dissipation ;  while  the  sixth  and 
last  chapter  gives  some  account  of  the  position  of  living 
beings  in  this  universe  of  Energy.  B.  S. 

Tlie  Owens  College,  Manchester, 
duffmt,  1873. 


CONTENTS. 


kote  to  the  american  edition, ▼ 

Preface, ▼" 

Chapter 

I. — What  is  Energy  ? 1 

II. — Mechanical  Energy  and  its  Change  into  Heat,       .        .  23 
III. — The  Forces   and   Energies   of  Nature  :   the   Law   of   Con- 
servation,           48 

IV. — Transmutations  of  Energy, 87 

V. — Historical  Sketch:   the  Dissipation  of  Energy,  .        .131 

VI. — The  Position  of  Life, 154 

Appendix 

I. — Correlation  of  Vital  with  Chemical  and  Physical  Forces. 
By  Joseph  Le  Conte,  Professor  of  Geology  and  Natural 
History  in  the  University  of  California,      ....  171 

II. — Correlation  of  Nervous  and  Mental  Forces.  By  Alexan- 
der Bain,  Professor  of  Logic  and  Mental  Philosophy  in 
the  University  of  Aberdeen, 205 


THE  CONSERVATION  OE  ENERGY. 


CHAPTER   L 

WHAT   IS   ENERGY f 

Our  Ignorance  of  Individuals. 

1.  Very  often  we  know  little  or  nothing  of  individuals, 
while  we  yet  possess  a  definite  knowledge  of  the  laws 
which  regulate  communities. 

The  Registrar- General,  for  example,  will  tell  us  that 
the  death-rate  in  London  varies  with  the  temperature  in 
such  a  manner  that  a  very  low  temperature  is  invariably 
accompanied  by  a  very  high  death-rate.  But  if  we  ask 
him  to  select  some  one  individual,  and  explain  to  us  in 
what  manner  his  death  was  caused  by  the  low  tempera- 
ture, he  will,  most  probably,  be  unable  to  do  so. 

Again,  we  may  be  quite  sure  that  after  a  bad  harvest 
there  will  be  a  large  importation  of  wheat  into  the 
country,  while,  at  the  same  time,  we  are  quite  ignorant 


2  THE   CONSERVATION  OF  ENERGY. 

of  the  individual  journeys  of  the  various  particles  of  flour 
that  go  to  make  up  a  loaf  of  bread. 

Or  yet  again,  we  know  that  there  is  a  constant  carriage 
of  air  from  the  poles  to  the  equator,  as  shown  by  the 
trade  Avinds,  and  yet  no  man  is  able  to  individualize 
a  particle  of  this  air,  and  describe  its  various  motions, 

'  2.  Nor  is  our  knowledge  of  individuals  greater  in  the 
domains  of  physical  science.  We  know  nothing,  or  next 
to  nothing,  of  the  ultimate  structure  and  properties  of 
matter,  whether  organic  or  inorgania 

No  doubt  there  are  certain  cases  where  a  larare  number 
of  particles  are  linked  together,  so  as  to  act  as  one 
individual,  and  then  we  can  predict  its  action — as,  for 
instance,  in  the  solar  system,  where  the  physical  astro- 
nomer is  able  to  foretell  with  great  exactness  the  posi- 
tions of  the  various  planets,  or  of  the  moon.  And  so,  in 
human  affairs,  we  find  a  large  number  of  individuals 
actino;  tocjether  as  one  nation,  and  the  sagacious  states- 
man  taking  very  much  the  place  of  the  sagacious 
astronomer,  with  regard  to  the  action  and  reaction  of 
vai'ious  nations  upon  one  another. 

But  if  we  ask  the  astronomer  or  the  statesman  to 
select  an  individual  particle  and  an  indi"\ddual  human 
being,  and  predict  the  motions  of  each,  we  shall  find  that 
both  will  be  completely  at  fault. 

5.  Nor  have  we  far  to  look  for  the  cause  of  their  igno- 
rance. A  continuous  and  restless,  nay,  a  very  complicated, 
activity  is  the  order  of  nature  throughout  all  her  indi- 


WHAT   IS   ENERGY?  3 

viduals,  whether  these  be  living  beings  or  inanimate 
particles  of  matter.  Existence  is,  in  truth,  one  continued 
fight,  and  a  great  battle  is  always  and  everywhere  raging, 
although  the  field  in  which  it  is  fought  is  often  com- 
pletely shrouded  from  our  view, 

4  Nevertheless,  although  we  cannot  trace  the  motions 
of  individuals,  we  may  sometimes  tell  the  result  of  the 
fight,  and  even  predict  how  the  day  will  go,  as  well  as 
specify  the  causes  that  contribute  to  bring  about  the 
issue. 

With  great  freedom  of  action  and  much  complication 
of  motion  in  the  individual,  there  are  yet  comparatively 
simple  laws  regulating  the  joint  result  attainable  by  the 
community. 

But,  before  proceeding  to  these,  it  may  not  be  out 
of  place  to  take  a  very  brief  survey  of  the  organic  and 
inorganic  worlds,  in  order  that  our  readers,  as  well  as 
ourselves,  may  realize  our  common  ignorance  of  the 
ultimate  structure  and  properties  of  matter. 

5.  Let  us  begin  by  referring  to  the  causes  which  bring 
about  disease.  It  is  only  very  recently  that"  we  have  be- 
gun to  suspect  a  large  number  of  our  diseases  to  be  caused 
by  organic  germs.  Now,  assuming  that  we  are  right  in 
this,  it  must  nevertheless  be  confessed  that  our  ignorance 
about  these  germs  is  most  complete.  It  is  perhaps 
doubtful  whether  we  ever  saw  one  of  these  organisms,* 

*  It  is  said  that  there  are  one  or  two  instances  where  the  microscope 
has  enlarged  them  into  Tisibility. 


i  THE   CONSERVATION   OF   ENERGY. 

while  it  is  certain  that  we  are  in  profound  ignorance  ot 
their  properties  and  habits. 

We  are  told  by  some  writers  *  that  the  very  air  we 
breathe  is  absolutely  teeming  with  germs,  and  that  we 
are  surrounded  on  all  sides  by  an  innumerable  array  of 
minute  organic  beings.  It  has  also  been  conjectured 
that  they  are  at  incessant  warfare  among  themselves,  and 
that  we  form  the  spoil  of  the  stronger  party.  Be  this  as 
it  may,  we  are  at  any  rate  intimately  bound  up  with, 
and,  so  to  speak,  at  the  mercy  of,  a  world  of  creatures,  of 
which  we  know  as  little  as  of  the  inliabitants  of  the 
planet  Mars. 

6.  Yet,  even  here,  with  profound  ignorance  of  the 
individual,  we  are  not  altogether  unacquainted  with  some 
of  the  habits  of  these  powerful  predatory  communities. 
Thus  we  know  that  cholera  is  eminently  a  low  level 
disease,  and  that  during  its  ravages  we  ought  to  pay 
particular  attention  to  the  water  we  drink.  This  is  a 
general  law  of  cholera,  which  is  of  the  more  importance 
to  us  because  we  cannot  study  the  habits  of  the  in- 
dividual organisms  that  cause  the  disease. 

Could  we  but  see  these,  and  experiment  upon  them,  we 
sliould  soon  acquire  a  much  more  extensive  knowledge  of 
their  habits,  and  perhaps  find  out  the  means  of  extirpat- 
ing the  disease,  and  of  preventing  its  recurrence. 

Again,  we  know  (thanks  to  Jenner)  that  vaccination 
will  prevent  the  ravages  of  small-pox,   but  in  this  in« 

*  See  Dr.  Angns  Smith  on  Air  and  Rain. 


WHAT   TS   ENERGY  ?  5 

stance  we  are  no  better  off  than  a  band  of  captives  who 
have  found  out  in  what  manner  to  mutilate  themselves, 
BO  as  to  render  them  uninteresting  to  their  victorious  foe. 

7.  But  if  our  knowledge  of  the  nature  and  habits  of 
organized  molecules  be  so  small,  our  knowledge  of  the 
ultimate  molecules  of  inorganic  matter  is,  if  possible,  still 
smaller.  It  is  only  very  recently  that  the  leading  men 
of  science  have  come  to  consider  their  very  existence  as  a 
settled  point. 

In  order  to  realize  what  is  meant  by  an  inorganic 
molecule,  let  us  take  some  sand  and  grind  it  into  smaller 
and  smaller  particles,  and  these  again  into  still  smaller. 
In  point  of  fact  we  shall  never  reach  the  superlative 
degree  of  smallness  by  this  operation — yet  in  our  imagi- 
nation we  may  suppose  the  sub-division  to  be  carried  on 
continuously,  always  making  the  particles  smaller  and 
smaller.  In  this  case  we  should,  at  last,  come  to  an 
ultimate  molecule  of  sand  or  oxide  of  silicon,  or,  in  other 
words,  we  should  arrive  at  the  smallest  entity  retaining 
all  the  properties  of  sand,  so  that  were  it  possible  to 
divide  the  molecule  further  the  only  result  would  be  to 
separate  it  into  its  chemical  constituents,  consisting  of 
silicon  on  the  one  side  and  oxygen  on  the  other. 

We  have,  in  truth,  much  reason  to  believe  that  sand, 
or  any  other  substance,  is  incapable  of  infinite  sub- 
division, and  that  all  we  can  do  in  grinding  down  a 
solid  lump  of  anything  is  to  reduce  it  into  lumps  similar 
to  the  original,  but  only  less  in  size,  each  of  these  small 


6  THE   CONSERVATION   OF   ENERGY. 

lumps  containing  probably  a  great  number  of  individual 
molecules. 

8.  Now,  a  drop  of  water  no  less  than  a  grain  of  sand  is 
built  up  of  a  very  great  number  of  molecules,  attached  to 
one  another  by  the  force  of  cohesion — a  force  which  is 
much  stronger  in  the  sand  than  in  the  water,  but  which 
nevertheless  exists  in  both.  And,  moreover,  Sir  William 
Thomson,  the  distinguished  physicist,  has  recently  ar- 
rived at  the  following  conclusion  with  recfard  to  the  size 
of  the  molecules  of  water.  He  imagines  a  single  drop  of 
water  to  be  magnified  until  it  becomes  as  large  as  the 
earth,  having  a  diameter  of  8000  miles,  and  all  the  mole- 
cules to  be  magnified  in  the  same  proportion ;  and  he 
then  concludes  that  a  single  molecule  will  appear,  under 
these  circumstances,  as  somewhat  larger  than  a  shot,  and 
somewhat  smaller  than  a  cricket  ball. 

'9.  Whatever  be  the  value  of  this  conclusion,  it  enables 
us  to  realize  the  exceedingly  small  size  of  the  individual 
molecules  of  matter,  and  renders  it  quite  certain  that  we 
shall  never,  by  means  of  the  most  powerful  microscope, 
succeed  in  making  visible  these  ultimate  molecules.  For 
our  knowledge  of  the  sizes,  shapes,  and  properties  of  such 
bodies,  we  must  always,  therefore,  be  indebted  to  indirect 
evidence  of  a  very  complicated  nature. 

It  thus  appears  that  we  know  little  or  nothing  about 
the  shape  or  size  of  molecules,  or  about  the  forces  which 
actuate  them ;  and,  moreover,  the  very  largest  masses  of 
the  universe  share  with  the  very  smallest  this  property 


WHAT   IS   ENERGY'^  7 

of  being  beyond  the  direct  scrutiny  of  the  human  senses 
— the  one  set  because  they  are  so  far  away,  and  the  other 
because  they  are  so  small. 

10.  Again,  these  molecules  are  not  at  rest,  but,  on  the 
contrary,  they  display  an  intense  and  ceaseless  energy  in 
their  motions.  There  is,  indeed,  an  uninterrupted  warfare 
going  on — a  constant  clashing  together  of  these  minute 
bodies,  which  are  continually  maimed,  and  yet  always 
recover  themselves,  until,  perhaps,  some  blow  is  struck 
sufficiently  powerful  to  dissever  the  two  or  more  simple 
atoms  that  go  to  form  a  compound  molecule.  A  new 
state  of  things  thenceforward  is  the  result. 

But  a  simple  elementary  atom  is  truly  an  immortal 
being,  and  enjoys  the  privilege  of  remaining  unaltered 
and  essentially  unaffected  amid  the  most  powerful  blows 
that  can  be  dealt  against  it — it  is  probably  in  a  state  of 
ceaseless  activity  and  change  of  form,  but  it  is  neverthe- 
less always  the  same. 

11.  Now,  a  little  reflection  will  convince  us  that  we 
have  in  this  ceaseless  activity  another  barrier  to  an  in- 
timate acquaintance  with  molecules  and  atoms,  for  even 
if  we  could  see  them  they  would  not  remain  at  rest 
sufficiently  long  to  enable  us  to  scrutinize  them. 

No  doubt  there  are  devices  by  means  of  which  we  can 
render  visi''ole,  for  instance,  the  pattern  of  a  quickly 
revolving  coloured  disc,  for  we  may  illuminate  it  by  a 
flash  of  electricity,  and  the  disc  may  be  supposed  to  bo 
stationary  during  the  extremely  short  time  of  the  flash 


8  THE   CONSERVATION    OF   ENERGY. 

But  we  cannot  say  the  same  about  molecules  and  atoms, 
for,  could  we  see  an  atom,  and  could  we  illuminate  it  Ly  a 
flash  of  electricity,  the  atom  would  most  probably  have 
vibrated  many  times  during  the  exceedingly  small  time 
of  the  flash.  In  fine,  the  limits  placed  upon  our  senses, 
with  respect  to  space  and  time,  equally  preclude  the 
possibility  of  our  ever  becoming  directly  acquainted  with 
these  exceedingly  minute  bodies,  which  are  nevertheless 
the  raw  materials  of  which  the  whole  universe  is  built. 

Action  and  Reaction,  Equal  and  OjJj^osite. 

12.  But  while  an  impenetrable  veil  is  drawn  over  the 
individual  in  this  warfare  of  clashing  atoms,  yet  we 
are  not  left  in  profound  ignorance  of  the  laws  which 
determine  the  ultimate  result  of  all  these  motions,  taken 
together  as  a  whola 

In  a  Vessel  of  Goldfish. 

Let  us  suppose,  for  instance,  that  we  have  a  glass  globe 
containing  numerous  goldfish  standing  on  the  table,  and 
delicately  poised  on  wheels,  so  that  the  slightest  push,  the 
one  way  or  the  other,  would  make  it  move.  These  gold- 
fish are  in  active  and  irregular  motion,  and  he  wculd  be 
a  very  bold  man  who  should  ventui'e  to  predict  the  move- 
ments of  an  individual  fish.  But  of  one  thing  we  may 
be  quite  certain  :  we  may  rest  assured  that,  notwith- 
standing all  the  irregular  motions  of  its  living  inhabitants 


WHAT    IS    ENERGY  '{  9 

the  globe  containing  the  goldfish   will   remain   at  rest 
upon  its  wheels. 

Even  if  the  table  were  a  lake  of  ice,  and  the  wheels 
were  extremely  delicate,  we  should  find  that  the  globe 
would  remain  at  rest.  Indeed,  we  should  be  exceedingly 
surprised  if  we  found  the  globe  going  away  of  its  own 
accord  from  the  one  side  of  the  table  to  the  other,  or  from 
the  one  side  of  a  sheet  of  ice  to  the  other,  in  consequence 
of  the  internal  motions  of  its  inhabitants.  Whatever  be 
the  motions  of  these  individual  units,  yet  we  feel  sure 
that  the  globe  cannot  move  itself  as  a  whole.  In  such  a 
system,  therefore,  and,  indeed,  in  every  system  left  to 
itself,  there  may  be  strong  internal  forces  acting  between 
the  various  parts,  but  these  actions  and  o^eactions  are 
equal  and  opposite,  so  that  while  the  small  parts,  whether 
visible  or  invisible,  are  in  violent  commotion  among  them- 
selves, yet  the  system  as  a  whole  will  remain  at  rest 

In  a  Rifle. 

13.  Now  it  is  quite  a  legitimate  step  to  pass  from  this 
instance  of  the  goldfish  to  that  of  a  rifle  that  has  just 
been  fired.  In  the  former  case,  we  imao-ined  the  eiobe, 
together  with  its  fishes,  to  form  one  system ;  and  in  the 
latter,  we  must  look  upon  the  rifle,  with  its  powder  and 
ball,  as  forming  one  system  also. 

Let  us  suppose  that  the  explosion  takes  place  through 
the  application  of  a  spark.  Although  this  spark  i3  an 
external  agent,  yet  if  we  reflect  a  little  we  shall  see  that 


10  THE   CONSERVATION    OF   ENERGY'. 

its  only  office  in  this  case  is  to  summon  up  the  internal 
forces  akeady  existing  in  the  loaded  rifle,  and  bring  them 
into  vigorous  action,  and  that  in  viiiue  of  these  internal 
forces  the  explosion  takes  place. 

The  most  prominent  result  of  this  explosion  is  the  out- 
rush  of  the  rifle  ball  with  a  velocity  that  may,  perhaps, 
carry  it  for  the  best  part  of  a  mile  before  it  comes  to 
rest ;  and  here  it  would  seem  to  us,  at  fii'st  sight,  that  the 
law  of  equal  action  and  reaction  is  certainly  broken,  for 
these  internal  forces  present  in  the  rifle  have  at  least  pro- 
pelled part  of  the  system,  namely,  the  rifle  ball,  with  a 
most  enormous  velocity  in  one  direction. 

li.  But  a  little  further  reflection  will  bring  to  liffht 
another  phenomenon  besides  the  out-rush  of  the  ball. 
It  is  well  known  to  all  sportsmen  that  when  a  fowling- 
piece  is  discharged,  there  is  a  kick  or  recoil  of  the  piece 
itself  against  the  .shoulder  of  the  sportsman,  which  he 
would  rather  get  rid  of,  but  which  we  most  gladly  wel- 
come as  the  solution  of  our  difliculty.  In  plain  terms, 
while  the  baU  is  projected  forwards,  the  rifle  stock  (if 
free  to  move)  is  at  the  same  moment  projected  backwards. 
To  fix  our  ideas,  let  us  suppose  that  the  rifle  stock  weighs 
100  ounces,  and  the  ball  one  ounce,  and  that  the  ball  is 
projected  forwards  with  the  velocity  of  1000  feet  per 
second ;  then  it  is  asserted,  by  the  law  of  action  and  re- 
action, that  the  rifle  stock  is  ,at  the  same  time  projected 
backwards  with  the  velocity  of  10  feet  per  second,  so 
that  the  mass  of  the  stock,  multiplied  by  its  velocity  of 


WHAT  IS  ENEEGY?  11 

recoil,  shall  precisely  equal  the  mass  of  the  ball,  multiplied 
by  its  velocity  of  projection.  The  one  product  forms  a 
measure  of  the  action  in  the  one  direction,  and  the  other 
of  the  reaction  in  the  opposite  direction,  and  thus  we 
see  that  in  the  case  of  a  rifle,  as  well  as  in  that  of  the 
globe  of  fish,  action  and  reaction  are  equal  and  opposite. 

In  a  Fallivg  Stone. 

15.  We  may  even  extend  the  law  to  cases  in  which  we 
do  not  perceive  the  recoil  or  reaction  at  alL  Thus,  if  I 
drop  a  stone  from  the  top  of  a  precipice  to  the  earth,  the 
motion  seems  all  to  be  in  one  direction,  while  at  the 
same  time  it  is  in  truth  the  result  of  a  mutual  attraction 
between  the  earth  and  the  stone.  Does  not  the  earth 
move  also  ?  We  cannot  see  it  move,  but  we  are  entitled 
to  assert  that  it  does  in  reality  move  upwards  to  meet 
the  stone,  although  quite  to  an  imperceptible  extent, 
and  that  the  law  of  action  and  reaction  holds  here  as 
truly  as  in  a  rifle,  the  only  difierence  being  that  in 
the  one  case  the  two  objects  are  rushing  together,  while 
in  the  other  they  are  rushing  apart.  Inasmuch,  how- 
ever, as  the  mass  of  the  earth  is  very  great  compared 
with  that  of  the  stone,  it  follows  that  its  velocity  must  be 
extremely  small,  in  order  that  the  mass  of  the  earth, 
multiplied  into  its  velocity  upwards,  shall  equal  the  mass 
of  the  stone,  multiplied  into  its  velocity  do^\^.l wards. 

16.  We  have  thus,  in  spite  of  our  ignorance  of  the 
ultimate  atoms  and   molecules   of  matter,  arrived   at  a 


12  THE   CONSERVATION  OF  ENERGY. 

general  law  which  regulates  the  action  of  internal  forces. 
We  see  that  these  forces  are  always  mutually  exerted,  and 
that  if  A  attracts  or  repels  B,  B  in  its  turn  attracts  or 
repels  A.  We  have  here,  in  fact,  a  very  good  instance  of 
that  kind  of  generalization,  which  we  may  arrive  at,  even 
in  spite  of  our  ignorance  of  individuals. 

But  having  now  arrived  at  this  law  of  action  and 
reaction,  do  we  know  all  that  it  is  desirable  to  know  ? 
have  we  got  a  complete  understanding  of  what  takes 
place  in  all  such  cases — for  instance,  in  that  of  the  rifle 
which  is  just  discharged  ?  Let  us  consider  this  point  a 
little  further. 

The  Rifie  further  considered. 

17.  We  define  quantity  of  motion  to  mean  the  product 
of  the  mass  by  the  velocity;  and  since  the  velocity  of 
recoil  of  the  rifle  stock,  multiplied  by  the  mass  of  the 
stock,  is  equal  to  the  velocity  of  projection  of  the  rifle 
ball,  multiplied  by  the  mass  of  the  ball,  we  conceive 
ourselves  entitled  to  say  that  the  quantity  of  motion,  or 
momentum,  generated  is  equal  in  both  directions,  so  that 
the  law  of  action  and  reaction  holds  here  also.  Never- 
theless, it  cannot  but  occur  to  us  that,  in  some  sense,  the 
motion  of  the  rifle  ball  is  a  very  different  thing  from  that 
of  the  stock,  for  it  is  one  thing  to  allow  the  stock  to 
recoil  against  your  shoulder  and  discharge  the  ball  into 
the  air,  and  a  very  different  thing  to  discharge  the  ball 
against  your  shoulder  and  allow  the  stock  to  fly  into  the 


WHAT   IS   ENERGY?  13 

air.  And  if  any  man  should  assert  the  absolute  equality 
between  the  blow  of  the  rifle  stock  and  that  of  the  rifle 
ball,  you  might  request  him  to  put  his  assertion  to  this 
pi-actical  test,  with  the  absolute  certainty  that  he  would 
decline.  Equality  between  the  two  ! — Impossible  !  Why, 
if  this  were  the  case,  a  company  of  soldiers  engaged  in 
war  would  suffer  much  more  than  the  enemy  against 
whom  they  fired,  for  the  soldiers  would  certainly  feel 
each  recoil,  while  the  enemy  would  suffer  fi-om  only  a 
small  proportion  of  the  bullets. 

The  Rifle  Ball  possesses  Energy. 

18.  Now,  what  is  the  meaning  of  this  great  difference 
between  the  two  ?  We  have  a  vivid  perception  of  a 
mighty  difference,  and  it  only  remains  for  us  to  clothe 
our  naked  impressions  in  a  properly  fitting  scientific 
garb. 

The  something  which  the  rifle  hall  possesses  in  contra- 
distinction to  the  rifle  stock  is  clearly  the  power  of 
overcoming  resistance.  It  can  penetrate  through  oak 
wood  or  tln"ough  water,  or  (alas  !  that  it  should  be  so 
often  tried)  through  the  human  body,  and  this  power  of 
penetration  is  the  distinguishing  characteristic  of  a 
substance  moving  with  very  great  velocity. 

19.  Let  us  define  by  the  term  energy  this  power  which 
the  rifle  ball  possesses  of  overcoming  obstacles  or  of  doing 
work.  Of  course  we  use  the  word  work  without  refer- 
ence to  the  moral  character  of  the  thing  done,  and  con- 


14  THE   CONSERVATION  OF  ENERGY. 

ceive  ourselves  entitled  to  sum  up,  with  perfect  propriety 
and  innocence,  the  amount  of  work  done  in  drilling  a  hole 
through  a  deal  board  or  through  a  man. 

20.  A  body  such  as  a  rifle  ball,  moving  with  very  great 
velocity,  has,  therefore,  energy,  and  it  requires  very  little 
consideration  to  perceive  that  this  energy  will  he  fro- 
portional  to  its  weight  or  Tnass,  for  a  ball  of  two  ounces 
moving  with  the  velocity  of  1000  feet  per  second  will  be 
the  same  as  two  balls  of  one  ounce  moving  with  this 
velocity,  but  the  energy  of  two  similarly  moving  ounce 
balls  wiU  manifestly  be  double  that  of  one,  so  that  the 
energy  is  proportional  to  the  weight,  if  we  imagine  that, 
meanwhile,  the  velocity  remains  the  same. 

21.  But,  on  the  other  hand,  the  energy  is  not  simply 
proportional  to  the  velocity,  for,  if  it  were,  the  energy  of 
the  rifle  stock  and  of  the  rifle  ball  would  be  the  same, 
inasmuch  as  the  rifle  stock  would  gain  as  much  by  its 
superior  mass  as  it  would  lose  by  its  infeiuor  velocity. 
Therefore,  the  energy  of  a  moving  body  increases  with  the 
velocity  more  quickly  than  a  simple  proportion,  so  that 
if  the  velocity  be  doubled,  the  energy  is  more  than 
doubled.  Now,  in  what  manner  does  the  energy  increase 
with  the  velocity  ?  That  is  the  question  we  have  now  to 
answer,  and,  in  doing  so,  we  must  appeal  to  the  familiar 
facts  of  everyday  observation  and  experience. 

'  22.  In  the  first  place,  it  is  well  kno^^^l  to  artiUerjTnen, 
that  if  a  ball  have  a  double  velocity,  its  penetrating 
power  or  energy  is   increased  nearly  fourfold,  so  that  it 


WHAT  IS  ENEEGY?  15 

will  pierce  through  four,  or  nea,rly  four,  times  as  many 
deal  boards  as  the  ball  ydth.  only  a  single  velocity — in 
other  words,  they  will  tell  us.  in  mathematical  language, 
tliat  the  energy  varies  as  the  square  of  the  velocity. 

Definition  of  Work. 

23.  And  now,  before  proceeding  further,  it  will  be 
necessary  to  tell  our  readers  how  to  measure  work  in  a 
strictly  scientific  manner.  We  have  defined  energy  to  be 
the  power  of  doing  work,  and  although  every  one  has  a 
general  notion  of  what  is  meant  by  work,  that  notion 
may  not  be  sufficiently  precise  for  the  purpose  of  this 
volume.  How,  then,  are  we  to  measure  work  ?  For- 
tunately, we  have  not  far  to  go  for  a  practical  means  of 
doing  this.  Indeed,  there  is  a  force  at  hand  which  enables 
us  to  accomplish  this  measurement  with  the  greatest  pre- 
cision, and  this  force  is  gravity.  Now,  the  first  operation 
in  any  kind  of  numerical  estimate  is  to  fix  upon  our  unit 
or  standard.  Thus  we  say  a  rod  is  so  many  inches  long, 
or  a  road  so  many  miles  long.  Here  an  inch  and  a  mile 
are  chosen  as  our  standards.  In  like  manner,  we  speak  of 
so  many  seconds,  or  minutes,  or  hours,  or  days,  or  years, 
choosing  that  standard  of  time  or  duration  which  is  most 
convenient  for  our  purpose.  So  in  like  manner  we  must 
choose  our  unit  of  work,  but  in  order  to  do  so  we  must 
first  of  all  choose  our  units  of  weight  and  of  length,  and 
for  these  we  will  take  the  kilogramme  and  the  Tnetre, 
these  being  the  units  of  the  metrical  system.     The  kilo- 

2 


16  THE   CONSERVATION   OF  ENERGY. 

gramme  corresponds  to  about  15,432  •  35  English  gi'alns, 
being  rather  more  than  two  pounds  avoirdupois,  and  tl.a 
metre  to  about  39  •  371  English  inches. 

Now,  if  we  raise  a  kilogramme  weight  one  metre  in 
vertical  height,  we  are  conscious  of  putting  forth  an 
effort  to  do  so,  and  of  being  resisted  in  the  act  by  tlie 
force  of  gravity.  In  other  words,  we  spend  energy  and 
do  work  in  the  process  of  raising  this  weight. 

Let  us  agi^ee  to  consider  the  energy  spent,  or  the  work 
done,  in  this  operation  as  one  unit  of  work,  and  let  us  call 
it  the  Idlogranimeire. 

24.  In  the  next  place,  it  is  very  obvious  that  if  we  raise 
the  kilogramme  two  metres  in  height,  we  do  two  units  of 
work — if  three  metres,  three  units,  and  so  on. 

And  again,  it  is  equally  obvious  that  if  we  raise  a 
weight  of  two  kilogrammes  one  metre  high,  we  likewise 
do  two  units  of  work,  while  if  we  raise  it  two  metres  hiffh, 
we  do  four  units,  and  so  on. 

From  these  examples  we  art  entitled  to  derive  the 
following  rule  : — Multiply  the  tveight  raised  {in  hilo- 
(jrammes)  by  the  vertical  height  (in  metres)  through  which 
it  is  raised,  and  the  result  ivill  be  the  work  done  (in 
kilogram/metres). 

Relation  between  Velocity  and  Energy. 

25.  Having  thus  laid  a  numerical  foundation  for  our 
superstructure,  let  us  next  proceed  to  iavestigate  the  rela- 
tion between  velocity  and  energy.     But  first  let  us  say  a 


WHAT  IS   ENERGY  ?  17 

few  words  about  velocity.  Tliis  is  one  of  the  fcAv  cases  in 
which  everyday  experience  will  aid,  rather  than  hinder, 
us  in  our  scientific  conception.  Indeed,  we  have  con- 
stantly before  us  the  example  of  bodies  moving  with 
variable  velocities. 

Thus  a  railway  train  is  approaching  a  station  and  is 
just  beginning  to  slacken  its  pace.  When  we  begin  to 
observe,  it  is  moving  at  the  rate  of  forty  miles  an 
\n\ur.  A  minute  afterwards  it  is  moving  at  the  rate 
of  twenty  miles  only,  and  a  minute  after  that  it  is  at 
rest.  For  no  two  consecutive  moments  has  this  train 
continued  to  move  at  the  same  rate,  and  yet  we  may 
say,  with  perfect  propriety,  that  at  such  a  moment 
the  train  was  moving,  say,  at  the  rate  of  thirty  miles 
an  hour.  We  mean,  of  course,  that  had  it  continued  to 
move  for  an  hour  with  the  speed  which  it  had  when 
we  made  the  observation,  it  would  have  gone  over 
thirty  miles.  We  know  that,  as  a  matter  of  fact,  it  did 
not  move  for  two  seconds  at  that  rate,  but  this  is  of  no 
consequence,  and  hardly  at  all  interferes  with  our  mental 
grasp  of  the  problem,  so  accustomed  are  we  all  to  cases 
of  variable  velocity. 

2G.  Let  us  now  imagine  a  kilogramme  weight  to  be 
shot  vertically  upwards,  with  a  certain  initial  velocity — 
let  us  say,  with  the  velocity  of  9  *  8  metres  in  one  second. 
Gravity  will,  of  course,  act  against  the  weight,  and 
continually  diminish  its  upward  speed,  just  as  in  the 
railway  train  the   break   was   constantly   reducing   the 


18  TUK   CONSERVATION  OF   ENERGY. 

velocity.  But  yet  it  is  very  easy  to  see  what  is  meant 
by  an  initial  velocity  of  9  '  8  metres  per  second ;  it  means 
that  if  gravity  did  not  interfere,  and  if  the  air  did  not 
resist,  and,  in  fine,  if  no  external  influence  of  any  kind 
were  allowed  to  act  upon  the  ascending  mass,  it  would  be 
found  to  move  over  9  '  8  metres  in  one  second. 

Now,  it  is  well  known  to  those  who  have  studied  the 
laws  of  motion,  that  a  body,  shot  upwards  mth  the 
velocity  of  9 '  8  metres  in  one  second,  will  be  brought 
to  rest  when  it  has  risen  4  •  9  metres  in  height.  If,  there- 
fore, it  be  a  kilogramme,  its  upward  velocity  will  have 
enabled  it  to  raise  itself  4  '  9  metres  in  height  against  the 
force  of  gravity,  or,  in  other  words,  it  wiR  have  done  4  "  9 
units  of  work  ;  and  we  may  imagine  it,  when  at  the  top  of 
its  ascent,  and  just  about  to  turn,  caught  in  the  hand  and 
lodged  on  the  top  of  a  house,  instead  of  being  allowed  to 
fall  again  to  the  ground.  We  are,  therefore,  entitled  to 
say  that  a  kilogramme,  shot  upwards  with  the  velocity 
of  9  ■  8  metres  per  second,  has  energy  equal  to  4*9,  inas- 
much as  it  can  raise  itself  4  •  9  metres  in  height. 

27.  Let  us  next  suppose  that  the  velocity  with  which 
the  kilogramme  is  shot  upwards  is  that  of  19  "6  metres 
per  second.  It  is  known  to  all  who  have  studied  dy- 
namics that  the  kilogramme  will  now  mount  not  only 
twice,  but  four  times  as  high  as  it  did  in  the  last  in- 
stance— in  other  words,  it  will  now  mount  19  "G  metres 
in  height. 

Evidently,  then,  in  accordance  with  our  principles   ot 


WHAT   IS   ENERGY  ?  1& 

measurement,  the  kilogramme  has  now  four  times  as 
much  energy  as  it  had  in  the  last  instance,  because  it 
can  raise  itself  four  times  as  high,  and  therefore  do  four 
times  as  much  work,  and  thus  we  see  that  the  energy  is 
increased  four  times  by  doubling  the  velocity. 

Had  the  initial  velocity  been  three  times  that  of  the 
first  instance,  or  29  "  4  metres  per  second,  it  might  in  Hke 
manner  be  shown  that  the  height  attained  would  have 
been  44  '  1  metres,  so  that  by  tripling  the  velocj  ty  the 
energy  is  increased  nine  times. 

28.  We  thus  see  that  whether  we  measui'e  the  energy 
of  a  moving  body  by  the  thickness  of  the  planks  through 
which  it  can  pierce  its  way,  or  by  the  height  to  which  it 
can  raise  itself  against  gravity,  the  result  arrived  at  is 
the  same.  We  find  tJie  energy  to  he  proportional  to 
the  square  of  the  velocity,  and  we  may  formularize 
our  conclusion  as  follows. : — 

Let  V  =  the  initial  velocity  expressed  in  metres  per 

v" 
second,  then  the  enero-y  in  kiloo-rammetres  =  vt^ — ^r  •      Of 
'  oj  o  19-6 

course,  if  the  body  shot  upwards  weighs  two  kilogrammes, 

then  everything  is  doubled,  if  three  kilogrammes,  tripled, 

and  so  on ;  so  that  finally,  if  we  denote  by  m  the  mass  of 

the  body  in  kilogrammes,  we  shall  have  the  energy  in  kilo- 

772/  V 

grammetres  =  .,,,    ^.     To  test  the  truth  of  this  formula, 
19  •  o 

we  have  only  to  apply  it  to  the  cases  described  in  Arts 

26  and  27. 


20  THE   CONSERVATION   OF   ENERGY. 

29.  We  may  further  illustrate  it  by  one  or  two 
examples.  For  instance,  let  it  be  required  to  find  the 
energy  contained  in  a  mass  of  five  kilogrammes,  shot  up- 
wards with  the  velocit}'-  of  20  metres  per  second. 

Here  we  have  m  =  5  and  v  =  20,  hence — 

5  r^oV        2000 
Energy  =  -^^  ^  -^-^  =  102  •  04  nearly. 

Again,  let  it  be  required  to  find  the  height  to  which  the 
mass  of  the  last  question  will  ascend  before  it  stops.  "We 
know  that  its  energy  is  102  •  01^,  and  that  its  mass  is  5. 
Dividing  102  •  04*  by  5,  we  obtain  20  •  408  as  the  height 
to  which  this  mass  of  five  kilogrammes  must  ascend  in 
order  to  do  work  equal  to  102  •  04  kilogrammetres. 

30.  In  what  we  have  said  we  have  taken  no  account 
either  of  the  resistance  or  of  the  buoyancy  of  the  atmo- 
sphere ;  in  fact,  we  have  supposed  the  experiments  to  be 
made  in  vacuo,  or,  if  not  in  vacuo,  made  by  means  of  a 
lieavy  mass,  like  lead,  which  wiU  be  very  little  influenced 
either  by  the  resistance  or  buoyancy  of  the  air. 

We  must  not,  however,  forget  that  if  a  sheet  of  paper, 
or  a  feather,  be  shot  upwards  with  the  velocities  men- 
tioned in  our  text,  they  wUl  certainly  not  rise  in  the  air 
to  nearly  the  height  recorded,  but  will  be  much  sooner 
brought  to  a  stop  by  the  very  great  resistance  which  they 
encounter  from  the  air,  on  account  of  their  great  suiface, 
combined  with  their  small  mass. 

On  the  other  hand,  if  the  substance  we  make  use  of  be 
a  large  light  bag  fiUed  with  hydi-ogen,  it  will  find  its  v^ay 


WHAT  IS   ENERGY  ?  21 

apwards  without  any  effort  on  our  part,  and  we  shall  cer- 
tamly  be  doing  no  work  by  carrying  it  one  or  more 
metres  in  height — it  will,  in  reality,  help  to  pull  us  up, 
instead  of  requiring  help  from  us  to  cause  it  to  ascend. 
In  fine,  what  we  have  said  is  meant  to  refer  to  the  force  of 
gravity  alone,  without  taking  into  account  a  resisting 
medium  such  as  the  atmosphere,  the  existence  of  which 
need  not  be  considered  in  our  present  calculations. 

31.  It  should  likewise  be  remembered,  that  while  the 
energy  of  a  moving  body  depends  upon  its  velocity,  it  is 
independent  of  the  direction  in  which  the  body  is 
moving.  We  have  supposed  the  body  to  be  shot  up- 
wards with  a  given  velocity,  but  it  might  be  sliot  hori- 
zontally with  the  same  velocity,  when  it  would  have 
precisely  the  same  energy  as  before.  A  cannon  ball,  if 
fired  vertically  upwards,  may  either  be  made  to  spend 
its  energy  in  raising  itself,  or  in  piercing  through  a 
series  of  deal  boards.  Now,  if  the  same  ball  be  fired 
horizontally  with  the  same  velocity  it  will  piei'ce  tln-ough 
the  same  number  of  deal  boards. 

In  fine,  direction  of  motion  is  of  no  consequence,  and 
the  only  reason  why  we  have  chosen  vertical  motion  is 
that,  in  this  case,  there  is  always  the  force  of  gravity 
steadily  and  constantly  opposing  the  motion  of  the  body, 
and  enabling  us  to  obtain  an  accurate  measure  of  the 
work  which  it  does  by  piercing  its  way  upwards  against 
this  force. 

32.  But  gravity  is  not  the  only  force,  and  we  might 


22  THE   CONSERVATION   OF   EiNERGY. 

measure  tlie  energy  of  a  moving  body  by  the  extent  to 
which  it  would  bend  a  powerful  spring  or  resist  the  at- 
traction of  a  powerful  magnet,  or,  in  fine,  we  might  make 
use  of  the  force  which  best  suits  our  purpose.  If  this 
force  be  a  constant  one,  we  must  measure  the  energy  of 
the  moving  body  by  the  space  which  it  is  able  to  traverse 
against  the  action  of  the  force — just  m,  in  the  case  of 
gravity,  we  measured  the  energy  of  the  body  by  the  space 
through  which  it  was  able  to  raise  itself  against  its  own 
weight: 

33.  We  must,  of  course,  bear  in  mind  that  if  this  force 
be  more  powerful  than  gravity,  a  body  moved  a  short 
distance  against  it  will  represent  the  expenditure  of  as 
much  energy  as  if  it  were  moved  a  greater  distance 
against  gravity.  In  fine,  we  must  take  account  both 
of  the  strength  of  the  force  and  of  the  distance  iJioved 
over  by  the  body  against  it  before  we  can  estimatt  in  an 
acciu'ate  matter  the  work  which  has  been  doufi. 


CHAPTER   II 

UECHANirAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT. 

Energy  of  Position.     A  Stone  high  up. 

84.  In  the  last  chapter  it  was  shown  what  is  meant 
by  energy,  and  how  it  depends  upon  the  velocity  of 
a  moving  body;  and  now  let  us  state  that  this 
same  energy  or  power  of  doing  work  may  neverthe- 
less be  possessed  by  a  body  absolutely  at  rest.  It 
will  be  remembered  (Art.  26)  that  in  one  case  where 
a  kilogramme  was  shot  vertically  upwards,  we  supposed 
it  to  be  caught  at  the  summit  of  its  flight  and  lodged  on 
the  top  of  a  house.  Here,  then,  it  rests  without  motion, 
but  yet  not  without  the  power  of  doing  work,  and  hence 
not  without  energy.  For  we  know  very  weU  that  if  we  let 
it  fall  it  wiU  strike  the  ground  with  as  much  velocity,  and, 
therefore,  with  as  much  energy,  as  it  had  when  it  was 
originally  projected  upwards.  Or  we  may,  if  we  choose, 
make  use  of  its  energy  to  assist  us  in  di-iving  in  a  pile,  or 
utilize  it  in  a  multitude  of  ways. 

In  its  lofty  position  it  is,  therefore,  not  without  energy, 
but  this  is  of  a  quiet  nature,  and  not  due  in  the  least  to 


24!  THE   CONSERVATION   OF   ENERGY. 

motion.  To  \\  hat,  then,  is  it  due  ?  We  reply — to  the 
poisition  which  the  kilogramme  occupies  at  the  top  of  the 
house.  For  just  as  a  body  in  motion  is  a  very  different 
thing  (as  regards  energy)  from  a  body  at  rest,  so  is  a  body 
at  the  top  of  a  house  a  very  different  thing  from  a  body 
at  the  bottom. 

To  illustrate  this,  we  may  suppose  that  two  men  of 
equal  activity  and  strength  are  fighting  together,  each 
having  his  pHe  of  stones  wdth  which  he  is  about  to  be- 
labour his  adversary.  One  man,  however,  has  secured  for 
himself  and  his  pile  an  elevated  position  on  the  top  of  a 
house,  while  his  enemy  has  to  remain  content  with  a 
position  at  the  bottom.  Now,  under  these  circumstances, 
you  can  at  once  tell  which  of  the  two  will  gain  the  day 
— evidently  the  man  on  the  top  of  the  house,  and  yet  not 
on  account  of  his  own  superior  energy,  but  rather  on 
account  of  the  energy  which  he  derives  from  the  elevated 
position  of  his  pile  of  stones.  We  thus  see  that  there 
is  a  kind  of  energy  derived  from  position,  as  well  as  a 
kind  derived  from  velocity,  and  we  shall,  in  future,  call 
the  former  energy  of  position,  and  the  latter  energy  of 
motion. 

A  Head  of  Water, 

35.  In  order  to  vary  our  illustration,  let  us  suppose 
there  are  two  mills,  one  with  a  large  pond  of  water  near 
it  and  at  a  high  level,  while  the  other  has  also  a  pond, 
but  at  a  lower  level  than  itself      We  need  hardly  ask 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     25 

which  of  the  two  is  likely  to  work — clearly  the  one 
with  the  pond  at  a  low  level  can  derive  from  it  no  advan- 
tage whatever,  while  the  other  may  use  the  high  level 
pond,  or  head  of  water,  as  this  is  sometimes  called,  to 
drive  its  wheel,  and  do  its  work.  There  is,  thus,  a  great 
deal  of  work  to  be  got  out  of  water  high  up — real  sub- 
stantial work,  such  as  grinding  corn  or  thrashing  it,  or 
turning  wood  or  sawing  it.  On  the  other  hand,  there  is  no 
work  at  all  to  be  got  from  a  pond  of  water  that  is  low  down. 

A  Cross-hoiu  bent.     A  Watch  ivound  up. 

36.  In  both  of  the  illustrations  novv^  given,  we  have 
used  the  force  of  gravity  as  that  force  against  which  we 
are  to  do  work,  and  in  virtue  of  which  a  stone  high  up, 
or  a  head  of  water,  is  in  a  position  of  advantage,  and  has 
the  power  of  doing  work  as  it  falls  to  a  lower  level.  But 
there  are  other  forces  besides  gravity,  and,  with  respect  to 
these,  bodies  may  be  in  a  position  of  advantage  and  be 
able  to  do  work  just  as  truly  as  the  stone,  or  the  head  of 
water,  in  the  case  before  mentioned. 

Let  us  take,  for  instance,  the  force  of  elasticity,  and 
consider  what  happens  in  a  cross-bow.  When  this  is 
bent,  the  bolt  is  evidently  in  a  position  of  advantage 
with  regard  to  the  elastic  force  of  the  bow ;  and  when 
it  is  discharged,  this  energy  of  position  of  the  bolt  is 
converted  into  energy  of  motion,  just  as,  when  a  stone  on 
the  top  of  a  house  is  allowed  to  fall,  its  energy  of  posi- 
tion is  converted  into  that  of  actual  motion. 


Si(J  THE   CONSERVATION   OF   ENERGY. 

In  like  manner  a  watch  wound  up  is  in  a  position  oi 
advantage  with  respect  to  the  elastic  force  of  the  main- 
spring, and  as  the  wheels  of  the  watch  move  this  is 
gradually  converted  into  energy  of  motion. 

Advantage  of  Position. 

37.  It  is,  in  fact,  the  fate  of  all  kinds  of  energy  of 
position  to  be  ultimately  converted  into  energy  of  motion. 

The  former  may  he  compared  to  money  in  a  bank,  or 
capital,  the  latter  to  money  which  we  are  in  the  act  of 
spending;  and  just  as,  when  we  have  money  in  a  bank,  we 
can  di-aw  it  out  whenever  we  want  it,  so,  in  the  case  of 
energy  of  position,  we  can  make  use  of  it  whenever  we 
please.  To  see  this  more  clearly,  let  us  compare  together 
a  watermiU  driven  by  a  head  of  water,  and  a  windmill 
driven  by  the  wind.  In  the  one  case  we  may  turn  on 
the  water  whenever  it  is  most  convenient  for  us,  but  in 
the  other  we  must  wait  until  the  wind  happens  to  blow. 
The  foiTner  has  aU  the  independence  of  a  rich  man ;  the 
latter,  all  the  obsequiousness  of  a  poor  one.  If  we  pursue 
the  analogy  a  step  further,  we  shall  see  that  the  great 
capitaHst,  or  the  man  who  has  acquired  a  lofty  position, 
is  respected  because  he  has  the  disposal  of  a  great 
quantity  of  energy ;  and  that  whether  he  be  a  nobleman 
or  a  sovereign,  or  a  general  in  command,  he  is  powerful 
only  from  having  something  which  enables  him  to  make 
use  of  the  services  of  others.  When  the  man  of  wealth 
pays  a  labouring  man  to  work  for  him,  he  is  in  truth 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     27 

converting  so  mucli  of  liis  energy  of  position  into  actual 
energy,  just  as  a  miller  lets  out  a  portion  of  his  head  of 
water  in  order  to  do  some  work  by  its  means. 

Transmutations  of  Visible  Energy. — A  Kilogramme 
shot  wpivards. 

38.  We  have  thus  endeavoured  to  show  that  there  is 
an  energy  of  repose  as  well  as  a  living  energy,  an  energy 
of  position  as  well  as  of  motion ;  and  now  let  us  trace 
the  changes  which  take  place  in  the  energy  of  a  weight, 
shot  vertically  upwards,  as  it  continues  to  rise.  It  starts 
with  a  certain  amount  of  energy  of  motion,  but  as  it 
ascends,  this  is  by  degrees  changed  into  that  of  position, 
until,  when  it  gets  to  the  top  of  its  flight,  its  energy  is 
entirely  due  to  position. 

To  take  an  example,  let  us  suppose  that  a  kilogramme 
is  projected  vertically  upwards  with  the  velocity  of  19  *  6 
metres  in  one  second.  According  to  the  formula  of  Art. 
28,  it  contains  19  '  6  units  of  energy  due  to  its  actual 
velocity. 

If  we  examine  it  at  the  end  of  one  second,  we  shall 
find  that  it  has  risen  14*7  metres  in  height,  and  has  now 
the  velocity  of  9*8.  This  velocity  we  know  (Art.  26) 
denotes  an  amount  of  actual  energy  equal  to  4<  •  9,  while 
the  height  reached  corresponds  to  an  energy  of  position 
equal  to  14  •  7.  The  kilogramme  has,  therefore,  at  this 
moment  a  total  energy  of  19  '6,  of  which  l-i  "7  units  are 
due  to  position,  and  4  •  9  to  actual  motion. 


2S  THE  CONSERVATION  OF  ENERGY. 

If  we  next  examine  it  at  tlie  end  of  another  second,  we 
shall  find  that  it  has  just  been  brought  to  rest,  so  that  its 
energy  of  motion  is  nil ;  nevertheless,  it  has  succeeded  in 
raising  itself  19*6  metres  in  height,  so  that  its  enei-gy  of 
position  is  19  "  6. 

There  is,  therefore,  no  disappearance  of  energy  during 
the  rise  of  the  kilogramme,  but  merely  a  gradual  change 
from  one  kind  to  another.  It  starts  with  actual  energy, 
and  this  is  gradually  changed  into  that  of  position ;  but 
if,  at  any  stage  of  its  ascent,  we  add  together  the  actual 
energy  of  the  kilogramme,  and  that  due  to  its  position, 
we  shall  find  that  their  sum  always  remains  the  same. 

39.  Precisely  the  reverse  takes  place  when  the  kilo- 
gramme begins  its  descent.  It  starts  on  its  downward 
journey  with  no  energy  of  motion  whatever,  but  with  a 
certain  amount  of  energy  of  position ;  as  it  falls,  its 
energy  of  position  becomes  less,  and  its  actual  energy 
greater,  the  sum  of  the  two  remaining  constant  through- 
out, until,  when  it  is  about  to  strike  the  grovmd,  its 
energy  of  position  has  been  entirely  changed  into  that 
of  actual  motion,  and  it  now  approaches  the  ground 
with  the  velocity,  and,  therefore,  with  the  energy,  which 
it  had  when  it  was  originally  projected  upwards. 

The  Inclined  Plane. 

40.  We  have  thus  traced  the  transmutations,  as  i-egards 
energy,  of  a  kilogramme  shot  vertically  upwards,  and 
allowed   to  fall  again   to  the   earth,  and   we  may  now 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.      21) 

vaiy  our  hypothesis  by  making  the  kilogramme  rise 
vertically,  but  descend  by  means  of  a  smooth  inclined 
plane  without  fiiction — imagine  in  fact,  the  kilogramme 
to  be  shaped  like  a  ball  or  roller,  and  the  plane  to  be 
perfectly  smooth.  Now,  it  is  well  known  to  all  students 
of  dynamics,  that  in  such  a  case  the  velocity  which  the 
kilogramme  has  when  it  has  reached  the  bottom  of  the 
plane  will  be  equal  to  that  which  it  would  have  had  if 
it  had  been  dropped  down  vertically  through  the  same 
height,  and  thus,  by  introducing  a  smooth  inclined  plane 
of  this  kind,  you  neither  gain  nor  lose  anything  as  regards 
energy. 

In  the  first  place,  you  do  not  gain,  for  think  what 
would  happen  if  the  kilogramme,  when  it  reached  the 
bottom  of  the  inclined  plane,  should  have  a  greater 
velocity  than  you  gave  it  originally,  when  you  shot  it  up. 
It  would  evidently  be  a  profitable  thing  to  shoot  up  the 
kilogTamme  vertically,  and  bring  it  down  by  means  of 
the  plane,  for  you  would  get  back  more  energy  than  you 
originally  spent  upon  it,  and  in  eveiy  sense  you  would 
be  a  gamer.  You  might,  in  fact,  by  means  of  appropriate 
apparatus,  convert  the  arrangement  into  a  perpetual 
motion  machine,  and  go  on  accumulating  energy  without 
limit — but  this  is  not  possible. 

On  the  other  hand,  the  inclined  plane,  unless  it  be 
rough  and  angular,  will  not  rob  you  of  any  of  the  energy 
-of  the  kilogramme,  but  will  restore  to  you  the  full  amount, 
when  once  the  bottom  has  been  reached.     Nor  does  it 


30 


THE   COlfSERVATION    OF   ENEEGY. 


matter  what  be  tlie  lengtli  or  shape  of  the  plane,  or 
whether  it  be  straight,  or  curved,  or  spiral,  for  in  all 
eases,  if  it  only  be  smooth  and  of  the  same  vertical 
height,  you  will  get  the  same  amount  of  energy  by  causing 
the  kilogramme  to  fall  from  the  top  to  the  bottom, 

41.  But  while  the  energy  remains  the  same,  the  time 
of  descent  will  vary  according  to  the  length  and  shape  of 
the  plane,  for  evidently  the  kilogramme  will  take  a  longer 
time  to  descend  a  very  sloping  plane  than  a  very  steep 
one.  In  fact,  the  sloping  plane  will  take  longer  to  gene- 
rate the  requisite  velocity  than  the  steep  one,  but  both 
fe?-.^JW/^-^  will  have  produced  the  same  result  as  regards 

energy,  when  once  the  kilogramme  has  arrived 

at  the  bottom. 


Functions  of  a  Machine. 

42.  Our  readers  are  now  beginning  to  per- 
ceive that  energy  cannot  be  created,  and  that 
by  no  means  can  we  coax  or  cozen  Dame 
Nature  into  giving  us  back  more  than  we  are 
entitled  to  get.  To  impress  this  fundamental 
principle  still  more  strongly  upon  our  minds, 
let  us  consider  in  detail  one  or  two  n.echan- 
ical  contrivances,  and  see  what  they  amount 
to  as  regards  energy. 

Let  us  begin  with  the  second  sj^stem  of 
pulleys.  Here  Ave  have  a  power  P  attached 
to  the   one    end    of  a   thread,  which  passes 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     31 

over  all  the  pulleys,  and  is  ultimately  attached,  by  its 
other  extremity,  to  a  hook  in  the  upper  or  fixed  block. 
The  weight  w  is,  on  the  other  hand,  attached  to  the 
lower  or  moveable  block,  and  rises  with  it.  Let  us 
suppose  that  the  pulleys  are  without  weight  and  the 
cords  without  friction,  and  that  w  is  supported  by  six 
cords,  as  in  the  figure.  Now,  when  there  is  equilibrium 
in  this  machine,  it  is  well  known  that  w  will  be  equal 
to  six  times  P  ;  that  is  to  say,  a  power  of  one  kilogramme 
will,  in  such  a  machine,  balance  or  support  a  weight  of 
six  kilogrammes.  If  P  be  increased  a  single  grain  more, 
it  will  overbalance  W,  and  P  will  descend,  while  W  will 
begin  to  rise.  In  such  a  case,  after  P  has  descended,  say 
six  metres,  its  weight  being,  say,  one  kilogramme,  it  has 
lost  a  quantity  of  energy  of  position  equal  to  six  units, 
since  it  is  at  a  lower  level  by  six  metres  than  it  was  before. 
We  have,  in  fact,  expended  upon  our  machine  six  units 
of  energy.  Now,  what  return  have  we  received  for  this 
expenditure  ?  Our  return  is  clearly  the  rise  of  W,  and 
mechanicians  will  tell  us  that  in  this  case  w  will  have 
risen  one  metre. 

But  the  weight  of  w  is  six  kilogrammes,  and  this 
having  been  raised  one  metre  represents  an  energy  of 
position  equal  to  six.  We  have  thus  spent  upon  our 
machine,  in  the  fall  of  P,  an  amount  of  energy  equal  to 
six  imits,  and  obtained  in  the  rise  of  w  an  equivalent 
amount  equal  to  six  units  also.  We  have,  in  truth, 
neither  gained  nor  lost  energy,  but  simply  changed  it 
into  a  form  more  convenient  for  our  use. 


32 


THE   CONSERVATION   OF   ENERGY. 


pillliliillilipaiiiwi 


FiR.  2. 


43.  To  impress  this  truth  still  more  strongly,  let  us 
take  quite  a  difierent  machine,  such  as  the  hydrostatic 
press.  Its  mode  of  action  will  be 
perceived  from  Fig.  2.  Here  we 
have  two  cylinders,  a  wide  and 
a  narrow  one,  which  are  con- 
nected together  at  the  bottom  by 
means  of  a  strong  tube.  Each  of 
these  cylinders  is  provided  with 
a  water-tight  piston,  the  space  beneath  being  filled  with 
water.  It  is  therefore  manifest,  since  the  two  cylinders 
are  connected  together,  and  since  water  is  incompressible, 
that  when  we  push  down  the  one  piston  the  other  will  be 
pushed  up.  Let  us  suppose  that  the  area  of  the  small  pis- 
ton is  one  square  centimetre,*  and  that  of  the  large  piston 
one  hundred  square  centimetres,  and  let  us  apply  a  weight 
of  ten  kilogrammes  to  the  smaller  piston.  Now,  it  is 
known,  from  the  laws  of  hydrostatics,  that  every  square 
centimetre  of  tlie  larger  piston  will  be  pressed  upwards 
with  the  force  of  ten  kilogrammes,  so  that  the  piston  will 
altogether  mount  with  the  force  of  1000  kilogrammes — 
that  is  to  say,  it  will  raise  a  weight  of  this  amount  as  it 
ascends. 

Here,  then,  we  have  a  machine  in  virtue  of  which  a 
pressure  of  ten  kilogrammes  on  the  small  piston  enables 
the  large  piston  to  rise   with  the  force  of  1000   kilo- 

•  That  is  to  say,  a  square  the  side  of  which  is  one  ceutimetre,  or  the 
liuudrcdth  ])art  of  a  metre. 


MECHAinCAL  ENERGY  AND  ITS  CHANGE  INTO  Hi: AT.     33 

^■ammes.  But  it  is  very  easy  to  see  that,  while  the 
small  piston  falls  one  metre,  the  large  one  will  only  rise 
one  centimetre.  For  the  quantity  of  water  under  the 
pistons  being  always  the  same,  if  this  be  pushed  down 
one  metre  in  the  narrow  cylinder,  it  will  only  rise  one 
centimetre  in  the  wide  one. 

Let  us  now  consider  what  we  gain  by  this  machine.  The 
power  of  ten  kilogrammes  applied  to  the  smaller  piston  is 
made  to  fall  through  one  metre,  and  this  represents  the 
amount  of  energy  which  we  have  expended  upon  our 
machine,  while,  as  a  return,  we  obtain  1000  kilogrammes 
raised  through  one  single  centimetre.  Here,  then,  as  in 
the  case  of  the  pulleys,  the  return  of  energy  is  precisely 
the  same  as  the  expenditure,  and,  provided  we  ignore 
friction,  we  neither  gain  nor  lose  anything  by  the  machine. 
All  that  we  do  is  to  transmute  the  energy  into  a  more 
convenient  form — what  we  gain  in  power  Ave  lose  in 
space ;  but  we  are  willing  to  sacrifice  space  or  quickness 
of  motion  in  ^order  to  obtain  the  tremendous  pressure  or 
force  wliich  we  get  by  means  of  the  hydrostatic  press. 

Priaci'ple  of  Virtual  Velocities. 

44.  These  illustrations  will  have  prepared  our  readers 
to  perceive  the  true  function  of  a  machine.  This  was 
first  clearly  defined  by  Galileo,  who  saw  that  in  any 
machine,  no  matter  of  what  kind,  if  we  raise  a  large 
weight  by  means  of  a  small  one,  it  will  be  found  that  the 
small  weight,  multiplied  into  the  space  through  which  it 


34 


THE   CONSERVATION   OF   ENERGY, 


is  lowered,  will  exactly  equal  the  large  weight,  multiplied 
into  that  through  wliich  it  is  raised. 

This  principle,  known  as  that  of  virtual  velocities, 
enables  us  to  perceive  at  once  our  true  position.  We  see 
that  the  world  of  mechanism  is  not  a  manufactory,  in 
which  energy  is  created,  but  rather  a  mart,  into  which 
we  may  bring  energy  of  one  kind  and  change  or  barter  it 
for  an  equivalent  of  another  kind,  that  suits  us  better — 
but  if  we  come  with  nothing  in  our  hand,  with  nothing 
we  shall  most  assuredly  return.  A  machine,  in  truth, 
does  not  create,  but  only  transmutes,  and  this  principle 
will  enable  us  to  tell,  without  further  knowledge  of 
mechanics,  what  are  the  conditions  of  equilibrium  of  any 
arrangement. 

For  instance,  let  it  be  required  to  find  those  of  a  lever, 
of  which  the  one  arm  is  three  times  as  Ions:  as  the  other. 
Here  it  is  evident  that  if  we  overbalance  the  lever  by  a 
single  grain,  so  as  to  cause  the  long  arm  with  its  power  to 
fall  down  while  the  short  one  with  its  weight  rises  up, 
then  the  long  arm  wiR  fall  three  inches  for  every  inch 
through  which  the  short  arm  rises;  and  hence,  to  make  up 
for  this,  a  single  kilogramme  on  the 
long  arm  will  balance  three  kilo- 
grammes on  the  short  one,  or  the 
power  will  be  to  the  weight  as  one 
is  to  three. 

45.  Or,  again,  let  us  take  the  in- 
clined plane  as  represented  in  Fig.  3, 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     o5 

tlere  we  liave  a  smooth  plane  and  a  weight  held  upon 
it  by  means  of  a  power  P,  as  in  the  figure.  Now, 
if  we  overbalance  P  by  a  single  grain,  we  shall  bring 
the  weight  w  from  the  bottom  to  the  top  of  the  plane. 
Bat  when  this  has  taken  place,  it  is  evident  that 
P  has  fallen  tlurough  a  vertical  distance  equal  to  the 
length  of  the  plane,  while  on  the  other  hand  \v  lias  only 
risen  through  a  vertical  distance  equal  to  the  height. 
Hence,  in  order  that  the  principle  of  virtual  velocities 
shall  hold,  we  must  have  P  multiplied  into  its  fall  equal 
to  w  multiplied  into  its  rise,  that  is  to  say, 

P  X  Length  of  plane  ==  w  X  Height  of  plane, 

P  Heio-ht. 

or  -    =  = — ^-^ 

w         Length. 

What  Friction  does. 

46.  The  two  examples  now  given  are  quite  sufficient  to 
enable  our  readers  to  see  the  true  function  of  a  machine, 
and  they  are  now  doubtless  disposed  to  acknowledge  that 
no  machine  will  give  back  more  energy  than  is  spent 
upon  it.  It  is  not,  however,  equally  clear  that  it  will 
not  give  back  less ;  indeed,  it  is  a  well-known  fact  that 
it  constantly  does  so.  For  we  have  supposed  our 
machine  to  be  without  friction — ^but  no  machine  is  with- 
out friction — and  the  consequence  is  that  the  available 
out-come  of  the  machine  is  more  or  less  diminished  by 
this  drawback.     Now,  unless  we  are  able  to  see  clearly 


36  THE  CONSERVATION  OF  ENERGY. 

what  part  friction  really  plays,  we  cannot  prove  tlie  con- 
servation of  energy.  We  see  clearly  enougli  that  energy 
cannot  be  created,  but  we  are  not  equally  sure  that  it 
cannot  be  destroyed;  indeed,  we  may  say  we  have 
apparent  grounds  for  believing  that  it  is  destroyed — 
that  is  our  present  position.  Now,  if  the  theory  of  the 
conservation  of  energy  be  true — that  is  to  say,  if  energy 
is  in  any  sense  indestructible — friction  will  prove  itself 
to  be,  not  the  destroyer  of  energy,  but  merely  the  con- 
verter of  it  into  some  less  apparent  and  perhaps  less 
useful  form. 

47.  We  must,  therefore,  prepare  ourselves  to  study 
what  friction  really  does,  and  also  to  recognize  energy 
in  a  form  remote  from  that  possessed  by  a  body  in  visible 
motion,  or  by  a  head  of  water.  To  friction  we  may 
add  percussion,  as  a  process  by  which  energy  is  appa- 
rently destroyed ;  and  as  we  have  (Art.  39)  considered 
the  case  of  a  kilogramme  shot  vertically  upwards,  de- 
monstrating that  it  will  ultimately  reach  the  ground 
with  an  energy  equal  to  that  with  which  it  was  shot 
upwards,  we  may  pursue  the  experiment  one  step  further, 
and  ask  what  becomes  of  its  energy  after  it  has  struck 
the  ground  and  come  to  rest  ?  We  may  vary  the  ques- 
tion by  asking  what  becomes  of  the  energy  of  the  smith's 
blow  after  his  hammer  has  struck  the  anvil,  or  what  of 
the  energy  of  the  cannon  ball  after  it  has  struck  the 
target,  or  what  of  that  of  the  railway  train  after  it  has 
been  stopped  by  friction  at  the  break-wheel  ?     All  these 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     o7 

are  cases  in  which  percussion  or  friction  appears  at  first 
sight  to  have  destroyed  visible  energy ;  but  before  pro- 
nouncing upon  this  seeming  destruction,  it  clearly  be- 
hoves us  to  ask  if  anything  else  makes  its  appearance  at 
the  moment  when  the  visible  energy  is  apparently 
destroyed.  For.  after  all,  energy  may  be  like  the  Eastern 
magicians,  of  whom  we  read  that  they  had  the  power  of 
changing  themselves  into  a  variety  of  forms,  but  were 
nevertheless  very  careful  not  to  disappear  altogether. 

When  Motion  is  destroyed,  Heat  ap])ears. 

48.  Now,  in  reply  to  the  question  we  have  put,  it  may 
be  confidently  asserted  that  whenever  visible  energy  is 
apparently  destroyed  by  percussion  or  friction,  something 
else  makes  its  appearance,  and  that  something  is  heat. 
Thus,  a  piece  of  lead  placed  upon  an  anvil  may  be  greatly 
heated  by  successive  blows  of  a  blacksmith's  hammer. 
The  collision  of  flint  and  steel  will  produce  heat,  and  a 
rapidly-moving  cannon  ball,  when  striking  against  an 
iron  target,  may  even  be  heated  to  redness.  Again,  with 
regard  to  friction,  we  know  that  on  a  dark  night  sparks 
are  seen  to  issue  from  the  break- wheel  which  is  stopping 
a  railway  train,  and  we  know,  also,  that  the  axles  of  rail- 
way carriages  get  alarmingly  hot,  if  they  are  not  well 
supplied  with  grease. 

Finally,  the  schoolboy  will  tell  us  that  he  is  in  the 
habit  of  rubbing  a  brass  button  upon  the  desk,  and  ap- 
plying it  to  the  back  of  his  neighbour's  hand,  and  that 


38  THE  CONSERVATION  OF  ENERGY. 

when  his  own  hand  has  been  treated  in  this  way,  he  has 
found  the  button  unmistakeably  hot. 

Heat  a  species  of  Motion. 

49.  For  a  long  time  this  appearance  of  heat  by  friction 
or  percussion  was  regarded  as  inexplicable,  because  it 
was  believed  that  heat  was  a  kind  of  matter,  and  it  was 
difficult  to  understand  where  all  this  heat  came  from. 
The  partisans  of  the  material  hypothesis,  no  doubt, 
ventured  to  suggest  that  in  such  processes  heat  might 
be  drawn  from  the  neighbouring  bodies,  so  that  the 
Caloric  (which  was  the  name  given  to  the  imaginary 
substance  of  heat)  was  squeezed  or  rubbed  out  of  them, 
according  as  the  process  was  percussion  or  friction.  But 
this  was  regarded  by  many  as  no  explanation,  even 
before  Sir  Humphry  Davy,  about  the  end  of  last  cen- 
tury, clearly  showed  it  to  be  untenable. 

50.  Davy's  experiments  consisted  in  rubbing  together 
two  pieces  of  ice  until  it  was  found  that  both  were 
nearly  melted,  and  he  varied  the  conditions  of  his  ex- 
periments in  such  a  manner  as  to  show  that  the  heat 
produced  in  this  case  could  not  be  abstracted  from  the 
neicrhbouring  bodies. 

51.  Let  us  pause  to  consider  the  alternatives  to  which 
we  are  diiven  by  this  experiment.  If  we  still  choose  to 
regard  heat  as  a  substance,  since  this  has  not  teen  taken 
from  the  surrounding  bodies,  it  must  necessarily  have 
been  created  in  the  process  of  friction.      But  if  we  choose 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     39 

to  regard  heat  as  a  species  of  motion,  we  have  a  simpler 
alternative,  for,  inasmuch  as  the  energy  of  visible  motion 
has  disappeared  in  the  process  of  friction,  we  may  sup- 
pose that  it  has  been  transformed  into  a  species  of  mole- 
cular motion,  which  we  call  heat ;  and  this  was  the  con- 
clusion to  which  Davy  came. 

52.  About  the  same  time  another  philosopher  was 
occupied  with  a  similar  experiment  Count  Rumford  was 
superintending  the  boring  of  cannon  at  the  arsenal  at 
Munich,  and  was  forcibly  struck  with  the  very  great 
amount  of  heat  caused  by  this  process.  The  source  of 
this  heat  appeared  to  him  to  be  absolutely  inexhaustible, 
and,  being  unwilling  to  regard  it  as  the  creation  of  a 
species  of  matter,  he  was  led  like  Davy  to  attribute  it  to 
motion. 

53.  Assuming,  therefore,  that  heat  is  a  species  of 
motion,  the  next  point  is  to  endeavour  to  comprehend 
what  kind  of  motion  it  is,  and  in  what  respects  it  is 
different  from  ordinary  visibK  motion.  To  do  this,  let  us 
imagine  a  railway  carriage,  full  of  passengers,  to  be  whirl- 
ing along  at  a  great  speed,  its  occupants  quietly  at  ease, 
because,  although  they  are  in  rapid  motion,  they  are  all 
moving  at  the  same  rate  and  in  the  same  direction.  Now, 
suppose  that  the  train  meets  with  a  sudden  check ; — a 
disaster  is  the  consequence,  and  the  quiet  placidity  of  the 
occupants  of  the  carriage  is  instantly  at  an  end. 

Even  if  we  suppose  that  the  carriage  is  not  broken  up 
and  its  occupants  killed,  yet  they  are  all  in  a  violent 


to  THE   CONSERVATION   OF   ENERGY. 

state  of  excitement ;  those  fronting  the  engine  are  driven 
with  force  against  their  opposite  neighbours,  and  are,  no 
doubt,  as  forcibly  repelled,  each  one  taking  care  of  him- 
self in  the  general  scramble.  Now,  we  have  only  to  sub- 
stitute particles  for  persons,  in  order  to  obtain  an  idea  of 
what  takes  place  when  percussion  is  converted  into  heat. 
We  have,  or  suppose  we  have,  in  this  act  the  same  violent 
collision  of  atoms,  the  same  thrusting  forward  of  A  upon 
B,  and  the  same  violence  in  pushing  back  on  the  part  of 
B — the  same  struggle,  confusion,  and  excitement — the 
only  difference  being  that  particles  are  heated  instead  of 
human  beings,  or  their  tempers. 

54.  We  are  bound  to  acknowledge  that  the  proof  which 
we  have  now  given  is  not  a  direct  one  ;  indeed,  we  have, 
in  our  first  chapter,  explained  the  impossibility  of  our 
ever  seeing  these  individual  particles,  or  watching  their 
movements ;  and  hence  our  proof  of  the  assertion  that 
heat  consists  in  such  movements  cannot  possibly  be  direct. 
We  cannot  see  that  it  does  so  consist,  but  yet  we  may 
feel  sure,  as  reasonable  beings,  that  we  are  right  in  our 
conjecture. 

In  the  argument  now  given,  we  have  only  two  alter- 
natives to  start  with — either  heat  must  consist  of  a 
motion  of  particles  or,  when  percussion  or  friction  is  con- 
verted into  heat,  a  peculiar  substance  called  caloric  must 
be  created,  for  if  heat  be  not  a  species  of  motion  it  must 
necessarily  be  a  species  of  matter.  Now,  we  have  pre- 
ferred to  consider  heat,  as  a  species  of  motion  to  the  alter- 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     41 

native  of  supposing  the  creation  of  a  peculiar  kind  of 
matter. 

55.  l^Tcvertlielcss,  it  is  desirable  to  have  something  to 
say  to  an  opponent  who,  rather  than  acknowledge  heat 
to  be  a  species  of  motion,  will  allow  the  creation- of  matter. 
To  such  an  one  we  would  say  that  innumerable  experi- 
ments render  it  certain  that  a  hot  body  is  not  sensibly 
heavier  than  a  cold  one,  so  that  if  heat  be  a  species  of 
matter  it  is  one  that  is  not  subject  to  the  law  of  gravity. 
If  we  burn  iron  wire  in  oxygen  gas,  we  are  entitled  to 
say  that  the  iron  combines  with  the  oxygen,  because  we 
know  that  the  product  is  heavier  than  the  original  iron 
by  the  very  amount  which  the  gas  has  lost  in  weight. 
But  there  is  no  such  proof  that  during  combustion  the 
iron  has  combined  with  a  substance  called  caloric,  and 
the  absence  of  any  such  proof  is  enough  to  entitle  us  to 
consider  heat  to  be  a  species  of  motion,  rather  than  a 
species  of  matter. 

Heat  a  Backward  and  Forward  Motion. 

5G.  We  shall  now  suppose  that  our  readers  have 
assented  to  our  proposition  that  heat  is  a  species  of 
motion.  It  is  almost  unnecessary  to  add  that  it  must 
be  a  species  of  backward  and  forward  motion ;  for 
nothing  is  more  clear  than  that  a  heated  substance  is 
not  in  motion  as  a  whole,  and  will  not,  if  put  upon  a 
table,  push  its  way  from  the  one  end  to  the  other. 

Tilathcmaticians  express  this  ])eculiarity  by  saying  that, 


42  THE  CONSERVATION  OF   ENERGY. 

although  there  is  violent  internal  motion  among  the  par- 
ticles, yet  the  centre  of  gravity  of  the  substance  remains 
at  rest ;  and  since,  for  most  purposes,  we  may  suppose  a 
Lody  to  act  as  if  concentrated  at  its  centre  of  gravity,  we 
may  say  that  the  body  is  at  rest. 

57.  Let  us  here,  before  proceeding  further,  borrow  an 
illustration  from  that  branch  of  physics  which  treats  of 
sound.  Suppose,  for  instance,  that  a  man  is  accurately 
balanced  in  a  scale-pan,  and  that  some  water  enters  his 
ear  J  of  course  he  will  become  heavier  in  consequence, 
and  if  the  balance  be  sufficiently  delicate,  it  will  exhibit 
the  difference.  But  suppose  a  sound  or  a  noise  enters 
his  car,  he  may  say  with  truth  that  something  has  entered, 
but  yet  that  something  is  not  matter,  nor  will  he  become 
one  whit  heavier  in  consequence  of  its  entrance,  and  he 
will  remain  balanced  as  before.  Now,  a  man  into  whose 
ear  sound  has  entered  may  be  compared  to  a  substance 
mto  which  heat  has  entered ;  we  may  therefore  suppose  a 
heated  body  to  be  similar  in  many  respects  to  a  sounding- 
body,  and  just  as  the  particles  of  a  sounding  body  move 
backwards  and  forwards,  so  we  may  suppose  that  the 
particles  of  a  heated  body  do  the  same. 

We  shall  take  another  opportunity  (Art.  1G2)  to  enlarge 
upon  this  likeness;  but,  meanwhile,  we  shall  suppose  that 
our  readers  perceive  the  analogy. 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     43 


MecJmnical  Equivalent  of  Heat 

58.  We  have  thus  come  to  the  conclusion  that  when 
any  heavy  body,  say  a  kilogramme  weight,  strikes  the 
ground,  the  visible  energy  of  the  kilogi^amme  is  changed 
into  heat ;  and  now,  having  established  the  fact  of  a  re- 
lationship between  these  two  forms  of  energy,  our  next 
point  is  to  ascertain  according  to  what  law  the  heating 
effect  depends  upon  the  height  of  fall.  Let  us,  for  in- 
stance, suppose  that  a  kilogramme  of  water  is  allowed  to 
drop  from  the  height  of  848  metres,  and  that  we  have 
the  means  of  confining  to  its  own.  particles  and  retaining 
there  the  heating  efiect  produced.  Now,  we  may  suppose 
that  its  descent  is  accomplished  in  two  stages ;  that,  first 
of  all,  it  falls  upon  a  platform  from  the  height  of  424 
metres,  and  gets  heated  in  consequence,  and  that  then 
the  heated  mass  is  allowed  to  fall  other  424  metres.  It 
is  clear  that  the  water  will  now  be  doubly  heated  ;  or,  in 
other  words,  the  heating  effect  in  such  a  case  will  be  pro- 
portional to  the  height  through  which  the  body  falls — that 
is  to  say,  it  will  be  proportional  to  the  actual  energy  which 
the  body  possesses  before  the  blow  has  changed  this  into 
heat.  In  fact,  just  as  the  actual  energy  represented  by  a 
fall  from  a  height  is  proportional  to  the  height,  so  is  the 
heating  effect,  or  molecular  energy,  into  which  the  actual 
energy  is  changed  proportional  to  the  height  also.  Having 
established  this  point,  we  now  wish  to  know  through 


^4 


THE  CONSEEVATION   OF  ENERGY. 


how  many  metres  a  kilogramme  of  water  must   fall  in 
order  to  be  heated  one  degree  centigrade. 

59.  For  a  precise  determination  of  this  important 
point,  we  are  indebted  to  Dr.  Joule,  of  Manchester,  who 
has,  perhaps,  done  more  than  any  one  else  to  put  the 
science  of  energy  upon  a  sure  foundation.  Dr.  Joule 
made  numerous  experiments,  with  the  view  of  arriving 
at  the  exact  relation  between  mechanical  energy  and 
heat ;  that  is  to  say,  of  determining  the  mechanical 
equivalent  of  heat.  In  some  of  the  most  important  of 
these  he  took  advantage  of  the  friction  of  fluids. 

60.  These  experiments  were  conducted  in  the  following 
manner.  A  certain  fixed  weight  was  attached  to  a  pulley, 
as  in  the  figure.     The  weight  had,  of  course,  a  tendency 


to  descend,  -and  hence  to  turn  the  pulley  round.  Tho 
pulley  had  its  axle  supported  upon  friction  wheels,  at  / 
and  /,  by  means  of  which  the   friction  caused  by  the 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     45 

movement  ot  the  pulley  was  very  much  reduced.  A 
string,  passing  over  the  circumference  of  the  pulley,  was 
wrapped  round  r,  so  that,  as  the  weight  descended,  the 
pulley  moved  round,  and  the  string  of  the  pulley  caused 
r  to  rotate  very  rapidly.  Now,  the  motion  of  the  axis  r 
was  conducted  within  the  covered  box  B,  where  there 
was  attached  to  r  a  system  of  paddles,  of  which  a  sketch 
is  given  in  figure ;  and  therefore,  as  r  moved,  these 
paddles  moved  also.  There  were,  altogether,  eight  sets 
of  these  paddles  revolving  between  four  stationary  vanes. 
If,  therefore,. the  box  were  full  of  liquid,  the  paddles  and 
the  vanes  together  would  churn  it  about,  for  these  sta- 
tionary vanes  would  prevent  the  liquid  being  carried 
along  by  the  paddles  in  the  direction  of  rotation. 

Now,  in  this  experiment,  the  weight  was  made  to 
descend  through  a  certain  fixed  distance,  which  was 
accurately  measured.  As  it  descended,  the  paddles  were 
set  in  motion,  and  the  energy  of  the  descending  weight 
was  thus  made  to  churn,  and  hence  to  heat  some  water 
contained  in  the  box  B.  When  the  weight  had  descended 
a  certain  distance,  by  undoing  a  small  peg  p,  it  could  be 
wound  up  again  without  moving  the  paddles  in  B,  and 
thus  the  heating  effect  of  several  falls  of  the  weight 
could  be  accumulated  until  this  became  so  great  as  to  be 
capable  of  being  accurately  measured  by  a  thermometer. 
It  ought  to  be  mentioned  that  great  care  was  taken  in 
these  experiments,  not  only  to  reduce  the  friction  of  the 
axles   of  the   pulley  as   much   as   possible,  but   also   to 


4:6  THE   CONSERVATION   OF   ENERGY. 

estimate  and  correct  for  this  friction  as  accurately  as 
possible  ;  in  fact,  every  precaution  was  taken  to  make  the 
experiment  successful. 

61.  Other  experiments  were  made  by  Joule,  in  some  of 
which  a  disc  was  made  to  rotate  against  another  disc  of 
cast-iron  pressed  against  it,  the  whole  arrangement  being 
immersed  in  a  cast-iron  vessel  filled  with  mercury. 
From  all  these  experiments,  Dr.  Joule  concluded  that  the 
quantity  of  heat  produced  by  friction,  if  we  can  preserve 
and  accurately  measure  it,  will  always  be  found  propor- 
tional to  the  quantity  of  work  expended  He  expressed 
this  proportion  by  stating  the  number  of  units  of  work  in 
kilogrammetres  necessary  to  raise  by  1°  C.  the  tempera- 
ture of  one  kilogi'amme  of  water.  This  was  424?,  as 
determined  by  his  last  and  most  complete  experiments; 
and  hence  we  may  conclude  that  if  a  kilogramme  of 
water  be  allowed  to  fall  through  42-t  metres,  and  if  its 
motion  be  then  suddenly  stopped,  sufiicient  heat  will  be 
generated  to  raise  the  temperature  of  the  water  through 
1°  C,  and  so  on,  in  the  same  proportion. 

62.  Now,  if  we  take  the  kilogrammetre  as  our  unit  of 
work,  and  the  heat  necessary  to  raise  a  kilogramme  of 
water  1°  C.  as  our  unit  of  heat,  this  proportion  may  be 
expressed  by  saying  that  one  heat  unit  is  equal  to  424 
units  of  work. 

This  number  is  frequently  spoken  of  as  the  mechanical 
equivalent  of  heat ;  and  in  scientific  treatises  it  is 
denoted  by  J.,  the  initial  of  Dr.  Joule's  name. 


MECHANICAL  ENERGY  AND  ITS  CHANGE  INTO  HEAT.     47 

63.  We  have  now  stated  the  exact  relationship  that 
subsists  between  mechanical  energy  and  heat,  and  before 
proceeding  further  with  proofs  of  the  great  law  of  con- 
servation, we  shall  endeavour  to  make  our  readers 
acquainted  with  other  varieties  of  energy,  on  the  ground 
that  it  is  necessary  to  penetrate  the  various  disguises 
that  our  magician  assumes  before  we  can  pretend  to 
explain  the  principles  that  actuate  him  in  his  trans- 
formatiojifi. 


t8  THE  CONSERVATION   OF   ENEIIGY. 


CHAPTER  III 

TEE  FORCES  AND  ENERGIES   OF  NATURE: 
THE  LAW  OF  CONSERVATION. 

61.  In  the  last  chapter  we  introduced  our  readers  to 
two  varieties  of  energy,  one  of  them  visible,  and  the  other 
invisible  or  molecular ;  and  it  will  now  be  our  duty  to 
search  through  the  whole  field  of  physical  science  for 
other  varieties.  Here  it  is  well  to  bear  in  mind  that  all 
energy  consists  of  two  kinds,  that  of  position  and  that  of 
actual  motion,  and  also  that  this  distinction  holds  for 
invisible  molecular  energy  just  as  truly  as  it  does  for  that 
which  is  visible.  Now,  energy  of  position  implies  a  body 
in  a  position  of  advantage  with  respect  to  some  force,  and 
hence  we  may  with  propriety  begin  our  search  by 
investigating  the  various  forces  of  nature. 

Gravitation. 

65.  The  most  general,  and  perhaps  the  most  important, 
of  these  forces  is  gravitation,  and  the  law  of  action  of  this 
force  may  be  enunciated  as  follows : — Every  particle  of 
the  universe  attracts  every  other  particle  with  a  force 


THE  FORCES  AND  ENERGIES  OF  NATURE.      41) 

depending  jointly  upon  the  mass  of  the  attracting  and 
of  the  attracted  particle,  and  varying  inversely  as  the 
square  of  distance  between  the  two.  A  little  explanation 
will  make  this  plain. 

Suppose  a  particle  or  system  of  particles  of  which 
the  mass  is  unity  to  be  placed  at  a  distance  equal  to  unity 
from  another  particle  or  system  of  particles  of  which  the 
mass  is  also  unity — the  two  wiU  attract  each  other.  Let  us 
agi'ee  to  consider  the  mutual  attraction  between  them 
equal  to  unity  also. 

Suppose,  now,  that  we  have  on  the  one  side  two  such 
systems  with  a  mass  represented  by  2,  and  on  the  other 
side  the  same  system  as  before,  with  a  mass  repre- 
sented by  unity,  the  distance,  meanwhile,  remaining 
unaltered.  It  is  clear  the  double  system  will  now  attract 
the  single  system  with  a  twofold  force.  Let  us  next 
suppose  the  mass  of  both  systems  to  be  doubled,  the 
distance  always  remaining  the  same.  It  is  clear  that  we 
shaU  now  have  a  fourfold  force,  each  unit  of  the  one 
system  attracting  each  unit  of  the  other.  In  like  manner, 
if  the  mass  of  the  one  system  is  2,  and  that  of  the  other 
3,  the  force  wiU  be  6.     We  may,  for  instance,  call  the 

components   of  the   one    system   A     A     and    those    of 

1       2' 
the  other  A  A  A  and  we  shall  have  A  pulled  towards 

345  1 

A    A    and   A    with   a  threefold    force,  and  A    puUed 
345'  2 

towards  A^   A,  and   A,  with  a   threefold  force,  making/ 

34  5 

altogether  a  force  equal  to  6. 


50  THE  CONSERVATION  OF  ENERGY. 

In  the  next  place,  let  the  masses  remain  unaltered,  but 
let  the  distance  between  them  be  doubled,  then  the  force 
will  be  reduced  fourfold.  Let  the  distance  be  tripled, 
then  the  force  will  be  reduced  ninefold,  and  so  on. 

66.  Gravitation  may  be  described  as  a  very  weak  force, 
capable  of  acting  at  a  distance,  or  at  least  of  appearing 
to  do  so.  It  takes  the  mass  of  the  whole  earth  to  pro- 
duce the  force  with  which  we  are  so  familiar  at  its 
surface,  and  the  presence  of  a  large  mass .  of  rock  or 
mountain  does  not  produce  any  appreciable  difference  in 
the  weight  of  any  substance.  It  is  the  gravitation  of  the 
earth,  lessened  of  coui'se  by  distance,  which  acts  upon 
the  moon  240,000  miles  away,  and  the  gravitation  of  the 
sun  influences  in  like  manner  the  eaith  and  the  vaiious 
other  planets  of  our  system.  ^ 

Elastic  Forces. 

67.  Elastic  forces,  although  in  their  mode  of  action 
very  different  from  gi-avity,  are  yet  due  to  visible 
arrangements  of  matter ;  thus,  when  a  cross-bow  is  bent, 
there  is  a  visible  change  produced  in  the  bow,  which,  as  a 
whole,  resists  this  bending,  and  tends  to  resume  its 
previous  position.  It  therefore  requires  energy  to  bend 
a  bow,  just  as  truly  and  visibly  as  it  does  to  raise  a 
weight  above  the  earth,  and  elasticity  is,  therefore,  aa 
truly  a  species  of  force  as  gravity  ia  We  shall  not  here 
attempt  to  discuss  the  various  ways  in  which  this  force 
may  act,  or  in  which  a  soUd  elastic  substance  wiU  resist 


THE  FORCES  AND  ENERGIES  OF  NATURE.      51 

all  attempts  to  deform  it ;  but  in  all  cases  it  is  clearly 
manifest  that  work  must  be  spent  upon  the  body,  and  the 
force  of  elasticity  must  be  encountered  and  overcome 
throughout  a  certain  space  before  any  sensible  deforma- 
tion can  take  place. 

Force  of  Cohesion. 

68.  Let  us  now  leave  the  forces  which  animate  large 
masses  of  matter,  and  proceed  to  discuss  those  which 
subsist  between  the  smaller  particles  of  which  these  large 
masses  are  composed.  And  here  we  must  say  one  word 
more  about  molecules  and  atoms,  and  the  distinction  we 
feel  ourselves  entitled  to  draw  between  these  very  small 
bodies,  even  although  we  shall  never  be  able  to  see  either 
the  one  or  the  other. 

In  our  first  chapter  (Art.  7)  we  supposed  the  continual 
sub-division  of  a  grain  of  sand  until  we  had  arrived  at 
the  smallest  entity  retaining  all  the  properties  of  sand 
— this  we  called  a  molecule,  and  nothing  smaller  than 
this  is  entitled  to  be  called  sand.  If  we  continue  this 
sub-division  further,  the  molecule  of  sand  separates  itself 
into  its  chemical  constituents,  consisting  of  silicon  on 
the  one  side,  and  oxygen  on  the  other.  Thus  we  arrive 
at  last  at  the  smallest  body  which  can  call  itself  silicon, 
and  the  smallest  which  can  call  itself  oxygen,  and  we 
have  no  reason  to  suppose  that  either  of  these  is  capable 
of  sub-division  into  something  else,  since  we  regard 
oxygen  and  silicon  as  elementary  or  simple  bodies.    Now, 


52  THE   CONSERVATIOX   OF   ENERGY. 

these  constituents  of  tlie  silicon  molecule  are  called  atoms, 
so  that  we  say  the  sand  molecule  is  divisible  into  atoms 
of  silicon  and  of  oxygen.  Furthermore,  we  have  strong 
reason  for  supposing  that  such  molecules  and  atoms  really 
exist,  but  into  the  arguments  for  their  existence  we  can- 
not now  enter — it  is  one  of  those  things  that  we  must 
ask  our  readers  to  take  for  granted. 

69.  Let  us  now  take  two  molecules  of  sand.  These, 
when  near  together,  have  a  very  strong  attraction  for 
each  other.  It  is,  in  truth,  this  attraction  which  renders 
it  difficult  to  break  up  a  crystalline  particle  of  sand  or 
rock  crystal.  But  it  is  only  exerted  when  the  molecules 
are  near  enough  together  to  form  a  homogeneous  crystal- 
line structure,  for  let  the  distance  between  them  be  some- 
what increased,  and  we  find  that  all  attraction  entirely 
vanishes.  Thus  there  is  little  or  no  attraction  between 
different  particles  of  sand,  even  although  they  are  very 
closely  packed  together.  In  like  manner,  the  integrity 
of  a  piece  of  glass  is  due  to  the  attraction  between  its 
molecules ;  but  let  these  be  separated  by  a  flaw,  and  it 
will  soon  be  found  that  this  very  small  increase  of  dis- 
tance greatly  diminishes  the  attraction  between  the  par- 
ticles, and  that  the  structure  will  now  faU  to  pieces  from 
the  slightest  cause.  Now,  these  examples  are  sufficient 
to  show  that  molecular  attraction  or  cohesion,  as  this  is 
called,  is  a  force  which  acts  very  powerfully  through  a 
certain  small  distance,  but  which  vanishes  altogether 
when   this   distance   becomes   perceptible.      Cohesion  is 


THE  FORCES  AND  ENERGIES  OF  NATURE.      53 

strongest  in  solids,  while  in  liquids  it  is  much  diminished, 
and  in  gases  it  may  be  said  to  vanish  altogether.  The 
molecules  of  gases  are,  in  truth,  so  far  away  from  one 
another,  as  to  have  little  or  no  mutual  attraction,  a  fact 
proved  hy  Dr.  Joule,  whose  name  was  mentioned  in  the 
last  chapter. 

Force  of  Chemical  Affinity. 
70.  Let  us  now  consider  the  mutual  forces  between 
atoms.  These  may  be  characterized  as  even  stronger 
than  the  forces  between  molecules,  but  as  disappearing 
still  more  rapidly  when  the  distance  is  increased.  Let 
us,  for  instance,  take  carbon  and  oxygen — two  substances 
which  are  ready  to  combine  together  to  form  carbonic 
acid,  whenever  they  have  a  suitable  opportunity.  In 
this  case,  each  atom  of  carbon  will  unite  with  two  of 
oxygen,  and  the  result  will  be  something  quite  different 
from  either.  Yet  under  ordinary  circumstances  carbon,  or 
its  representative,  coal,  will  remain  unchanged  in  the 
presence  of  oxygen,  or  of  atmospheric  air  containing 
oxygen.  There  will  be  no  tendency  to  combine  together, 
because  although  the  particles  of  the  oxygen  would  appear 
to  be  in  immediate  contact  with  those  of  the  carbon, 
yet  the  nearness  is  not  sufficient  to  permit  of  chemical 
affinity  acting  with  advantage.  When,  however,  the 
nearness  becomes  sufficient,  then  chemical  affinity  begins 
to  operate.  We  have,  in  fact,  the  familiar  act  of  com- 
bustion,  and,  as  its  consequence,  the  chemical  union  of  the 


54!  THE  CONSERVATION   OF  ENERGY. 

carbon  or  coal  with  the  oxygen  of  the  air,  carljonic  acid 
being  the  result.  Here,  then,  we  have  a  very  powerful 
force  acting  only  at  a  very  small  distance,  which  we 
name  chemical  afinity,  inasmuch  as  it  represents  the 
attraction  exerted  between  atoms  of  different  bodies  in 
contradistinction  to  cohesion,  which  denotes  the  attraction 
between  molecules  of  the  same  body. 

71.  If  we  regard  gTavitation  as  the  representative  of 
forces  that  act  or  appear  to  act,  at  a  distance,  we  may 
regard  cohesion  and  chemical  affinity  as  the  representa- 
tives of  those  forces  which,  although  very  powerful,  only 
act  or  appear  to  act  through  a  very  small  interval  of 
distance. 

A  little  reflection  will  show  us  how  inconvenient  it 
would  be  if  gravitation  diminished  very  rapidly  with  the 
distance ;  for  then  even  supposing  that  the  bond  which 
retains  us  to  the  earth  were  to  hold  good,  that  which 
retains  the  moon  to  the  earth  might  vanish  entirely,  as 
well  as  that  which  retains  the  earth  to  the  sun,  and  the 
consequences  would  be  far  from  pleasant.  Reflection 
will  also  show  us  how  inconvenient  it  would  be  if 
chemical  affinity  existed  at  all  distances ;  if  coal,  for 
instance,  were  to  combine  with  oxygen  without  the  ap- 
plication of  heat,  it  would  greatly  alter  the  value  of  this 
fuel  to  mankind,  and  would  materially  check  the  progress 
of  human  industry. 


THE  FORCES  AND  ENERGIES  OF  NATURE.  65 

RemarJcs  on  Molecular  and  Atomic  Forces. 

72.  Now,  it  is  important  to  remember  that  we  must 
tieat  cohesion  and  chemical  affinity  exactly  in  the  same 
way  as  gravity  has  been  treated;  and  just  as  we  have 
energy  of  position  with  respect  to  gravity,  so  may  we 
have  as  truly  a  species  of  energy  of  position  with 
respect  to  cohesion  and  chemical  affinity.  Let  us 
begin  with  cohesion. 

73.  We  have  hitherto  regarded  heat  as  a  peculiar 
motion  of  the  molecules  of  matter,  without  any  reference 
to  the  force  which  actuates  these  molecules.  But  it  is 
a  well-known  fact  that  bodies  in  general  expand  when 
heated,  so  that,  in  virtue  of  this  expansion,  the  molecules 
of  a  body  are  driven  violently  apart  against  the  force  of 
cohesion.  Work  has  in  truth  been  done  against  this 
force,  just  as  truly  as,  when  a  kilogTamme  is  raised  from 
the  earth,  work  is  done  against  the  force  of  gravity. 
When  a  substance  is  heated,  we  may,  therefore,  suppose 
that  the  heat  has  a  twofold  office  to  perform,  part  of  it 
going  to  increase  the  actual  motions  of  the  molecules, 
and  part  of  it  to  separate  these  molecules  from  one 
another  against  the  force  of  cohesion.  Thus,  if  I  swing 
round  horizontally  a  weight  (attached  to  my  hand  by 
an  elastic  thread  of  india-rubber),  my  energy  will  be 
spent  in  two  ways — first  of  all,  it  will  be  spent  in  com- 
municating a  velocity  to  the  weight ;  and,  secondly,  in 
stretching   the    india-rubber  string,   by  means   of   the 


5G  THE   CONSERVATION   OF   ENERGY. 

centrifugal  tendency  of  the  weight.  Work  will  be  done 
against  the  elastic  force  of  the  string,  as  well  as  spent 
in  increasing  the  motion  of  the  weight. 

Now,  something  of  this  kind  may  be  taking  place 
when  a  body  is  heated,  for  we  may  very  well  suppose 
heat  to  consist  of  a  vertical  or  circular  motion,  the  ten- 
dency of  which  would  be  to  drive  the  particles  asunder 
against  the  force  of  cohesion.  Part,  therefore,  of  the 
energy  of  heat  will  be  spent  in  augmenting  the  motion, 
and  part  in  driving  asunder  the  particles.  We  may, 
however,  suppose  that,  in  ordinary  cases,  the  great  pro- 
portion of  the  energy  of  heat  goes  towards  increasing 
the  molecular  motion,  rather  than  in  doing  work  against 
the  force  of  cohesion. 

7-i.  In  certain  cases,  however,  it  is  probable  that  the 
greater  part  of  the  heat  applied  is  spent  in  doing  work 
a2:ainst  molecular  forces,  instead  of  increasing-  the 
motions  of  molecules. 

Thus,  when  a  solid  melts,  or  when  a  liquid  is  rendered 
gaseous,  a  considerable  amount  of  heat  is  spent  in  the 
process,  which  does  not  become  sensible,  that  is  to  say, 
does  not  affect  the  thermometer.  Thus,  in  order  to  melt 
a  kilogramme  of  ice,  heat  is  required  sufficient  to  raise 
a  kilogramme  of  water  through  80°  C,  and  yet,  when 
melted,  the  water  is  no  warmer  than  the  ice.  We  ex- 
press this  fact  by  saying  that  the  latent  heat  of  water 
is  80.  Again,  if  a  kilogramme  of  water  at  100°  be  con- 
verted entii"ely  into  steam,  as  much  heat  is  required  as 


THE   FORCES   AND   ENEllGIES   OF   NATURE.  57 

would  raise  the  water  through  537°  C,  or  537  kilogrammes 
of  water  through  one  degree;  hut  yet  the  steam  is  no 
hotter  than  the  water,  and  we  express  this  fact  by  saying 
that  the  latent  heat  of  steam  is  537.  Now,  in  both  of 
these  instances  it  is  at  least  extremely  probable  that 
a  large  portion  of  the  heat  is  spent  in  doing  work  against 
the  force  of  cohesion  ;  and,  more  especially,  when  a  fluid 
is  converted  into  a  gas,  we  know  that  the  molecules  are 
in  that  process  separated  so  far  from  one  another  as  to 
lose  entirely  any  trace  of  mutual  force.  We  may,  there- 
fore, conclude  that  although  in  most  cases  the  greater 
portion  of  the  heat  applied  to  a  body  is  spent  in  in- 
creasing its  molecular  motion,  and  only  a  small  part  in 
doing  work  against  cohesion,  yet  when  a  solid  melts,  or 
a  liquid  vapori^ies,  a  large  portion  of  the  heat  required  is 
not  improbably  spent  in  doing  work  against  molecular 
forces.  But  the  energy,  though  spent,  is  not  lost,  for 
when  the  liquid  again  freezes,  or  when  the  vapour  again 
condenses,  this  energy  is  once  more  transformed  into  the 
shape  of  sensible  heat,  just  as  when  a  stone  is  dropped 
from  the  top  of  a  house,  its  energy  of  position  is  trans- 
formed once  more  into  actual  energy. 

75.  A  single  instance  will  suffice  to  give  our  readers  a 
notion  of  the  strength  of  molecular  forces.  If  a  bar  of 
wrought  iron,  whose  temperature  is  10°  C  above  that 
of  the  surrounding  medium,  be  tightly  secured  at  its 
extremities,  it  will  draw  these  together  with  a  force  of  at 
least  one  ton  for  each  square  inch  of  section.     In  some 


58  THE  CONSERVATION  OF  ENERGY. 

cases  where  a  building  has  shown  signs  of  bulging  out- 
wards, iron  bars  have  been  placed  across  it,  and  secured 
while  in  a  heated  state  to  the  walls.  On  cooling,  the 
ii'on  contracted  with  great  force,  and  the  walls  were 
thereby  pulled  together. 

76.  We  are  next  brought  to  consider  atomic  forces,  or 
those  which  lead  to  chemical  union,  and  now  let  us  see 
how  these  are  iniiuenced  by  heat.  We  have  seen  that 
heat  causes  a  separation  between  the  molecules  of  a 
body,  that  is  to  say,  it  increases  the  distance  between 
two  contiguous  molecules,  but  we  must  not  suppose  that, 
meanwhile,  the  molecules  themselves  are  left  unaltered. 

The  tendency  of  heat  to  cause  separation  is  not  confined 
to  inci'easing  the  distance  between  molecules,  but  acts 
also,  no  doubt,  in  increasing  the  distance  between  parts 
of  the  same  molecule :  in  fact,  the  energy  of  heat  is  spent 
in  pulling  the  constituent  atoms  asunder  against  the  force 
of  chemical  affinity,  as  well  as  in  pulling  the  molecules 
asunder  against  the  force  of  cohesion,  so  that,  at  a  very 
high  temperature,  it  is  probable  that  most  chemical  com- 
pounds would  be  decomposed,  and  many  are  so,  even  at  a 
very  moderate  heat. 

Thus  the  attraction  between  oxygen  and  silver  is  so 
slight  that  at  a  comparatively  low  temperature  the  oxide 
of  silver  is  decomposed.  In  like  manner,  limestone,  or 
carbonate  of  lime,  is  decomposed  when  subjected  to  the 
heat  of  a  lime-kiln,  carbonic  acid  being  given  off,  while 
quick-lime  remains  behind.     Now,  in  separating  hetero- 


THE  FORCES  AND  ENERGIES  OF  NATURE.      69 

geneous  atoms  against  the  powerful  force  of  chemical 
affinity,  work  is  done  as  truly  as  it  is  in  separating  molecules 
from  one  another  against  the  force  of  cohesion,  or  in  separ- 
ating a  stone  from  the  earth  against  the  force  of  gravity, 

77.  Heat,  as  we  have  seen,  is  very  frequently  influential 
in  performing  this  separation,  and  its  energy  is  spent  in 
so  doing;  but  other  energetic  agents  produce  chemical 
decomposition  as  well  as  heat.  For  instance,  certain  rays 
of  the  sun  decompose  carbonic  acid  into  carbon  and 
oxygen  in  the  leaves  of  plants,  and  their  energy  is  spent 
in  the  process  ;  that  is  to  say,  it  is  spent  in  pulling 
asunder  two  such  powerfully  attracting  substances  against 
the  affinity  they  have  for  one  another.  And,  again,  the 
electric  current  is  able  to  decompose  certain  substances, 
and  of  course  its  energy  is  spent  in  the  process. 

Therefore,  whenever  two  powerfully  attracting  atoms 
are  separated,  energy  is  spent  in  causing  this  separation 
as  truly  as  in  separating  a  stone  from  the  earth,  and 
when  once  the  separation  has  been  accomplished  we  have 
a  species  of  energy  of  position  just  as  truly  as  we  have  in 
a  head  of  water,  or  in  a  stone  at  the  top  of  a  house. 

78.  It  is  this  chemical  separation  that  is  meant  when 
we  speak  of  coal  as  a  source  of  energy.  Coal,  or  carbon, 
has  a  great  attraction  for  oxygen,  and  whenever  heat  is 
applied  the  two  bodies  unite  together.  Now  oxygen,  as 
it  exists  in  the  atmosphere,  is  the  common  inheritance  of 
all,  and  if,  in  addition  to  this,  some  of  us  possess  coal  in 
our  cellars,  or  in  pits,  we  have  thus  secured  a  store  of 


GO  THE  CONSERVATION  OF  ENERGY. 

energy  of  position  which  we  can  draw  upon  with  more 
facility  than  if  it  were  a  head  of  water,  for,  although  we 
can  di'aw  upon  the  energy  of  a  head  of  water  whenever 
we  choose,  yet  we  cannot  carry  it  about  with  us  from 
place  to  place  as  we  can  with  coaL  We  thus  perceive 
that  it  is  not  the  coal,  by  itself,  that  forms  the  source  of 
energy,  but  tliis  is  due  to  the  fact  that  we  have  coal,  or 
carbon,  in  one  place,  and  oxygen  in  another,  while  we 
have  also  the  means  of  causing  them  to  unite  with  each 
other  whenever  we  wish.  If  there  were  no  oxj^gen  in 
the  air,  coal  by  itself  would  be  of  no  value. 

Electricity :  its  Properties. 

79.  Our  readers  have  now  been  told  about  the  force 
of  cohesion  that  exists  between  molecules  of  the  same 
body,  and  also  about  that  of  chemical  affinity  existing 
between  atoms  of  different  bodies.  Now,  heterogeneity 
is  an  essential  element  of  this  latter  force — there  must 
be  a  difference  of  some  kind  before  it  can  exhibit  itself — 
and  under  these  cii'cumstances  its  exhibitions  are  fre- 
quently characterized  by  very  extraordinary  and  interest- 
ing phenomena. 

We  allude  to  that  peculiar  exhibition  arising  out  of  the 
forces  of  heterogenous  bodies  which  we  call  electricity, 
and,  before  proceeding  further,  it  may  not  be  out  of  place 
to  give  a  short  sketch  of  the  mode  of  action  of  this  very 
!cr>ysterious,  but  most  interesting,  agent. 

80.  The  science  of  electricity  is  of  very  ancient  origin  ; 


TtiE  Forces  and  ejsekuies  of  natuiie.  01 

but  its  beginning  was  very  small.  For  a  couple  of  thou- 
sand years  it  made  little  or  no  progress,  and  then,  during 
the  course  of  little  more  than  a  century,  developed  into 
the  ffiant  which  it  now  is.  The  ancient  Greeks  were 
aware  that  amber,  when  rubbed  with  silk,  had  the  pro- 
perty of  attracting  light  bodies ;  and  Dr.  Gilbert,  about 
three  hundred  years  ago,  showed  that  many  other  things, 
such  as  sulphur,  sealing-wax,  and  glass,  have  the  same 
property  as  amber. 

In  the  progress  of  the  science  it  came  to  be  known 
that  certain  substances  are  able  to  carry  away  the 
peculiar  influence  produced,  while  others  are  unable  to 
do  so ;  the  former  are  called  conductors,  and  the  latter 
non-conductors,  or  insulators,  of  electricity.  To  make 
the  distinction  apparent,  let  us  take  a  metal  rod,  having 
a  glass  stem  attached  to  it,  and  rub  the  glass  stem  with 
a  piece  of  silk,  care  being  taken  that  both  silk  and  glass 
are  warm  and  dry.  We  shall  find  that  the  glass  has  now 
acquired  the  property  of  attracting  little  bits  of  paper,  or 
elder  pith ;  but  only  where  it  has  been  rubbed,  for  the 
peculiar  influence  acquired  by  the  glass  has  not  been  able 
to  spread  itself  over  the  surface. 

If,  however,  we  take  hold  of  the  glass  stem,  and  rub 
the  metal  rod,  we  may,  perhaps,  produce  the  same  pro- 
perty in  the  metal,  but  it  will  spread  over  the  whole,  not 
confining  itself  to  the  part  rubbed.  Thus  we  perceive 
that  metal  is  a  conductor,  while  glass  is  an  insulator,  or 
Qon-couductor,  of  electricity. 


62 


THE   CONSERVATION   OF  ENEEGT. 


81.  We  would  next  observe  that  this  influence  is  of 
tioo  kinds.     To  prove  this,  let  us  perform  the  following 

experiment.  Let  us  suspend 
a  small  pith  ball  by  a  very- 
slender  silk  thread,  as  in  Fig.  5. 
Next,  let  us  rub  a  stick  of 
warm,  dry  glass  with  a 
piece  of  warm  silk,  and  with 
this  excited  stick  touch  the 
pith  ball.  The  pith  ball,  after 
being  touched,  will  be  repelled 
by  the  excited  glass.  Let  us 
next  excite,  in  a  similar  man- 
ner, a  stick  of  dry  sealing-wax  with  a  piece  of  warm,  dry 
flannel,  and  on  approaching  this  stick  to  the  pith  ball  it 
will  attract  it,  although  the  ball,  in  its  present  state,  is 
repelled  by  the  excited  glass. 

Thus  a  pith  ball,  touched  by  excited  glass,  is  repelled 
by  excited  glass,  but  attracted  by  excited  sealing-wax. 

In  like  manner,  it  might  be  shown  that  a  pith  ball, 
touched  by  excited  sealing-wax,  will  be  afterwards  re- 
pelled by  excited  sealing-wax,  but  attracted  by  excited 
glass. 

Now,  what  the  excited  glass  did  to  the  pith  ball,  was 
to  communicate  to  it  jDart  of  its  own  influence,  after 
which  the  ball  was  repelled  by  the  glass ;  or,  in  other 
words,  bodies  charged  with  similar  electricities  repel  one 
another. 


THE  FORCES  AND  ENERGIES  OF  NATURK      63 

Again,  since  the  pith  ball,  when  charged  with  the  elec- 
tricity from  glass,  was  attracted  to  the  electrified  seaUng- 
wax,  we  conclude  that  bodies  charged  xvith  unlike  elec- 
tricities attract  one  another.  The  electricity  from  glass 
is  sometimes  called  vitreous,  and  that  from  sealing-wax 
resinous,  electricity,  but  more  frequently  the.  former  is 
known  as  positive,  sjidi  the  latter  as  negative,  oiQciviciij — 
it  being  understood  that  these  words  do  not  imply  the 
possession  of  a  positive  nature  by  the  one  influence,  or 
of  a  negative  nature  by  the  other,  but  are  merely  terms 
employed  to  express  the  apparent  antagonism  which 
exists  between  the  two  kinds  of  electricity. 

82.  The  next  point  worthy  of  notice  is  that  whenever 
one  electricity  is  produced,  jiist  as  much  is  'produced  oj 
an  opposite  description.  Thus,  in  the  case  of  glass 
excited  by  silk,  we  have  positive  electricity  developed 
upon  the  glass,  while  we  have  also  negative  electricity 
developed  upon  the  silk  to  precisely  the  same  extent.  And, 
again,  when  sealing-wax  is  rubbed  with  flannel,  we  ha,ve 
negative  electricity  developed  upon  the  sealing-wax,  and 
just  as  much  positive  upon  the  flannel. 

83,  These  facts  have  given  rise  to  a  theory  of  elec- 
tricity, or  at  least  to  a  method  of  regarding  it,  which,  if 
not  absolutely  correct,  seems  yet  to  unite  together  the 
various  phenomena.  According  to  this  hypothesis,  a 
neutral,  unexcited  body  is  supposed  to  contain  a  store 
of  the  two  electricities  combined  together,  so  that  when- 
ever such  a  body  is  excited,  a  separation  is  produced 

4 


Gi;  THE   CONSERVATION   OF   ENERGY. 

between  the  two.  The  phenomena  which  we  have 
described  are,  therefore,  due  to  this  electrical  separation, 
and  inasmuch  as  the  two  electricities  have  a  great  affinity 
for  one  another,  it  requires  the  expenditure  of  energy  to 
produce  this  separation,  just  as  truly  as  it  does  to  separate 
a  stone  from  the  earth. 

84.  Now,  it  is  worthy  of  note  that  electrical  separa- 
tion is  only  produced  when  heterogeneous  bodies  are 
rubbed  together.  Thus,  if  flannel  be  rubbed  upon  glass, 
we  have  electricity ;  but  if  flannel  be  rubbed  upon  glass 
covered  with  flannel,  we  have  none.  In  like  manner,  if 
silk  be  rubbed  upon  sealing-wax  covered  with  silk,  or,  in 
fine,  if  two  portions  of  the  same  substance  be  rubbed 
together,  we  have  no  electricity. 

On  the  other  hand,  a  very  slight  difference  of  texture 
is  sometimes  sufficient  to  produce  electrical  separation. 
Thus,  if  two  pieces  of  the  same  silk  ribbon  be  rubbed 
together  lengthwise,  we  have  no  electricity ;  but  if  they 
be  rubbed  across  each  other,  the  one  is  positively,  and  the 
other  negatively,  electrified. 

In  fact,  this  element  of  heterogeneity  is  an  all  impor- 
tant one  in  electrical  development,  and  this  leads  us  to 
conjecture  that  electrical  attraction  may  probably  be 
regarded  as  peculiarly  allied  to  that  force  which  we  call 
chemical  ajffinify.  At  any  rate,  electricity  and  chemical 
affinity  are  only  manifested  between  bodies  that  are,  in 
Bome  respects,  dissimilar. 

85.  The  following  is  a  list  of  bodies  arranged  according 


THE  FORCES  AND  ENERGIES  OF  NATURE.      05 

to  the  electricity  which  they  develop  when  rubbed  to- 
gether, each  substance  being  positively  electrified  when 
rubbed  with  any  substance  beneath  it  in  the  list. 

1.  Cat's  skin.  8.  Resin. 

2.  Flannel  9.  Metals. 
S.  Ivory.                                   10.  Sulphur. 

4.  Glass.  11.  Caoutchouc. 

5.  Silk.  12.  Gutta-percha 

6.  Wood.  13.  Gun-cotton. 

7.  Shellac. 

Thus,  if  resin  be  rubbed  with  cat's  skin,  or  with 
flannel,  the  cat's  skin  or  flannel  will  be  positively,  and 
the  resin  negatively,  electrified ;  while  if  glass  be  rubbed 
with  silk,  the  glass  will  be  positively,  and  the  silk  nega- 
tively, electrified,  and  so  on. 

86.  It  is  not  our  purpose  here  to  describe  at  length  the 
electrical  Tnachine,  but  we  may  state  that  it  consists  of 
two  parts,  one  for  generating  electricity  by  means  of  the 
friction  of  a  rubber  against  glass,  and  another  consisting 
of  a  system  of  brass  tubes,  of  considerable  surface,  sup- 
ported on  glass  stems,  for  collecting  and  retaining  the 
electricity  so  produced.  This  latter  part  of  the  machine 
is  called  its  i^rinic  conductor. 

Electric  Induction. 

87.  Let  us  now  suppose  that  we  have  set  in  action  a 
machine  of  this  kind,  and  accumulated  a  considerable 


66 


THE  CONSERVATION   OF  ENERGY. 


quantity  of  positive  electricity  in  its  prime  conductor  at 
A.     Let  us  next  take  two  vessels^  B  and  c,  made  of  brass, 


Fig-.  C. 

supported  on  glass  stems.  These  two  vessels  are  sup- 
posed to  be  in  contact,  but  at  the  same  time  to  be 
capable  of  being  separated  from  one  another  at  their 
middle  point,  where  the  line  is  drawn  in  Fig.  6. 
Now  let  us  cause  B  and  c  to  approach  A  together.  At 
first,  B  and  C  are  not  electrified,  that  is  to  say,  their  two 
electricities  are  not  separated  from  each  other,  but  are 
mixed  together;  but  mark  what  will  happen  as  they 
are  pushed  towards  A.  The  positive  electricity  of  A  will 
decompose  the  two  electricities  of  B  and  c,  attracting  the 
negative  towards  itself,  and  repelling  the  positive  as  far 
away  as  possible.  The  disposition  of  electricities  will, 
therefore,  be  as  in  the  figure.  If  we  now  pull  c  away 
from  B,  we  have  obtained  a  quantity  of  positive  elec- 
tricity on  C,  by  help  of  the  original  electricity  which  was 
in  A ;  in  fact,  we  have  made  use  of  the  original  stock  or 
electrical  capital  in  A,  in  order  to  obtain  positive  elec- 


THE  FORCES  AND  ENERGIES  OF  NATURE.      C7 

tricity  in  c,  without,  liowever,  diminishing  the  amount 
of  our  original  stock.  Now,  this  distant  action  or  help, 
rendered  by  the  original  electricity  in  separating  that  of 
B  and  C,  is  called  electric  induction. 

88.  The  experiment  may,  however,  he  performed  in  a 
somewhat  different  manner — we  may  allow  B  and  c  to 
remain  together,  and  gradually  push  them  nearer  to  A. 
As  B  and  c  approach  A,  the  separation  of  their  electricities 
will  become  greater  and  greater,  until,  when  A  and  B  are 
only  divided  by  a  small  thickness  of  air,  the  two  opposite 
electricities  tlien  accumulated  will  have  sufficient  strength 
to  rush  together  through  the  air,  and  unite  with  each, 
other  by  means  of  a  spark. 

89.  The  principle  of  induction  may  be  used  with  ad- 
vantage, when  it  is  wished  to  accumulate  a  large  quantity 
of  electricity. 

In  this  case,  an  instrument  called  a  Leyden  jar  is  very 
frequently  employed.  It  consists  of  a  glass  jar,  coated 
inside  and  outside  with  tin  foil,  as  in 
Fig.  7.  A  brass  rod,  having  a  knob  at 
the  end  of  it,  is  connected  metallically 
with  the  inside  coating,  and  is  kept  in 
its  place  by  being  passed  through  a 
cork,  which  covers  the  mouth  of  the 
jar.  We  have  thus  two  metallic 
coatings    which   are    not    electrically  Fig.  7. 

coimected  with  one  another.     Now,  in  order  to   charge 
a  jar   of  this   kind,    let    the     outside    coating  be   con- 


C8  THE   CONSEKVATION  OF  ENERGY. 

nected  by  a  chain  with  the  earth,  while  at  the  same 
time  positive  electricity  from  the  prime  conductor  of 
an  electrical  machine  is  communicated  to  the  inside  knob. 

The  positive  electricity  will  accumulate  on  the  inside 
coating  with  which  the  knob  is  connected.  It  will  then 
decompose  the  two  electricities  of  the  outside  coating, 
driving  the  positive  electricity  to  the  earth,  and  there 
dissipating  it,  but  attracting  the  negative  to  itself  There 
A\dll  thus  be  positive  electricity  on  the  inside,  and 
nesjative  on  the  outside  coating.  These  two  electricities 
may  be  compared  to  two  hostile  armies  watching  each 
other,  and  very  anxious  to  get  together,  while,  however, 
they  are  separated  from  one  another  by  means  of  an 
insurmountable  obstacle.  They  will  thus  remain  facing 
each  other,  and  at  their  posts,  while  each  side  is,  mean- 
while, being  recruited  by  the  same  operation  as  before. 
We  may  by  this  means  accumulate  a  vast  quantity  of 
opposite  electricities  on  the  two  coatings  of  such  a  jar, 
and  they  will  remain  there  for  a  long  time,  especially  if 
the  surrounding  atmosphere  and  the  glass  surface  of  the 
jar  be  quite  dry.  When,  however,  electric  connection  of 
any  kind  is  made  between  the  two  coatings,  the  elec- 
tricities rush  together  and  unite  with  one  another  in  the 
shape  of  a  spark,  while  if  the  human  body  be  the  instru- 
ment of  connecting  them  a  severe  shock  will  be  felt 

90.  It  would  thus  appear  that,  when  two  bodies 
charged  with  opposite  electricities  are  brought  near 
each  other,  the  two  electricities  rush  together,  forming 


THE  FORCES  AND  ENERGIES  OF  NATURE.      G9 

a  current,  and  the  ultimate  result  is  a  spark.  Now, 
this  spark  implies  heat,  and  is,  in  truth,  nothing  else 
than  small  particles  of  intensely  heated  matter  of  some 
kind.  We  have  here,  therefore,  first  of  all,  the  conversion 
of  electrical  separation  into  a  current  of  electricity,  and, 
secondly,  the  conversion  of  this  current  into  heat.  In 
this  case,  however,  the  current  lasts  only  a  very  small 
time ;  the  discharge,  as  it  is  called,  of  a  Leyden  jar  being 
probably  accomplished  in  ^^th  of  a  second 

The  ElectriG  Current 

91.  In  other  cases  we  have  electrical  currents  which, 
although  not  so  powerful  as  that  produced  by  discharging 
a  Leyden  jar,  yet  last  longer,  and  are,  in  fact,  continuous 
instead  of  momentary. 

We  may  see  a  similar  difference  in  the  case  of  visible 
energy.  Thus  we  might,  by  means  of  gunpowder,  send 
up  in  a  moment  an  enormous  mass  of  water;  or  we 
might,  by  means  of  a  fountain,  send  up  the  same  mass 
in  the  course  of  time,  and  in  a  very  much  quieter 
manner.  We  have  the  same  sort  of  difference  in  electrical 
discharges,  and  having  spoken  of  the  rushing  together  of 
two  opposite  electricities  by  means  of  an  explosion  and 
a  spark,  let  us  now  speak  of  the  eminently  quiet  and 
effective  voltaic  current,  in  which  we  have  a  continuous 
coming  together  of  the  same  two  agents. 

92.  It  is  not  our  object  here  to  give  a  complete  de- 
scription, either   historical   or   scientific,    of  the   voltaic 


70 


THE   CONSERVATION   OF  ENERGY. 


battery,  but  ratlier  to  give  such  an  account  as  will 
enable  our  readers  to  understand  what  the  arrangement 
is,  and  what  sort  of  effect  it  produces ;  and  with  this 
object  we  shall  at  once  proceed  to  describe  the  battery 
of  Grove,  which  is  perhaps  the  most  efficacious  of  all  the 
various  arrangements  for  the  purpose  of  producing  an 
electric  current.  In  this  battery  we  have  a  number  oi 
cells  connected  toire- 
ther,  as  in  Fig.  8, 
which  shows  a  battery 
of  three  cells.  Each 
cell   consists    of   two 

vessels,  an  outer  and  Fig.  8.  "^ 

an  inner  one ;  the  outer  vessel  being  made  of  glass 
or  ordinary  stone  ware,  while  the  inner  one  is  made 
of  unglazed  porcelain,  and  is  therefore  porous.  The 
outer  vessel  is  filled  with  dilute  sulphuric  acid,  and  a 
plate  of  amalgamated  zinc — that  is  to  say,  of  metallic 
zinc  having  its  outer  surface  brightened  with  mercury, — 
is  immersed  in  this  acid.  Again,  in  the  inner  or  porous 
vessel  we  have  strong  nitric  acid,  in  which  a  plate  of 
platinum  foil  is  immersed,  this  being  at  the  same  time  elec- 
trically connected  with  the  zinc  plate  of  the  next  outer 
vessel,  by  means  of  a  clamp,  as  in  the  figure.  Both  metals 
must  be  clean  where  they  are  pressed  together,  that  is  to 
say,  the  true  metallic  surfaces  of  both  must  be  in  contact. 
Finally,  a  wire  is  metallically  connected  with  the  plati- 
num of  the  left-hand  cell,  and  a  similar  wire  with  tho 


THE   FOKCES  AND  ENERGIES   OF   NATURE.  71 

rxnc  of  the  rio-ht-hand  cell,  and  these  connectincc  wires 
ought,  except  at  their  extremities,  to  be  covered  over 
with  gutta-percha  or  thread.  The  loose  extremities  of 
these  wires  are  called  the  2)0168  of  the  battery. 

93.  Let  us  now  suppose  that  we  have  a  battery  con- 
taining a  good  many  cells  of  this  description,  and  let  the 
whole  arrangement  be  insulated,  by  being  set  upon  glass 
supports,  or  otherwise  separated  from  the  earth.  If  now 
we  test,  by  appropriate  methods,  the  extremity  of  the 
wire  comiected  with  the  left-hand  platinum  plate,  it  will 
be  found  to  be  charged  with  positive  electricity,  while 
the  extremity  of  the  other  wire  will  be  found  charged 
with  negative  electricity. 

94  In  the  next  place,  if  we  connect  these  poles  of  the 
battery  with  one  another,  the  two  electricities  will  rush 
together  and  unite,  or,  in  other  words,  there  will  be  an 
electric  current ;  but  it  will  not  be  a  momentary  but  a 
continuous  one,  and  for  some  time,  provided  these  poles 
are  kept  together,  a  current  of  electricity  wall  pass  through 
the  wires,  and  indeed  throuo-h  the  whole  arran^'ement, 
including  the  cells. 

The  direction  of  the  current  will  be  such  that  positive 
electricity  may  he  supposed  to  pass  from  the  zinc  to  the 
platinuon,  through  the  liquid;  and  bach  again  through 
the  wire,  from  the  platinum  at  the  left  hand,  to  the  zinc 
at  the  right ;  in  fact,  to  go  in  the  direction  indicated  by 
the  arrow-head. 

95.  Thus  we  have  two  things.     In  the  first  place,  before 


72  THE   CONSERVATION   OF   ENERGY. 

the  two  terminals,  or  poles,  have  been  brought  together, 
we  have  them  charged  with  opposite  electricities ;  and, 
secondly,  when  once  they  have  been  brought  together,  we 
have  the  production  of  a  continuous  current  of  electricity. 
Now,  this  current  is  an  energetic  agent,  in  proof  of  which 
we  shall  proceed  to  consider  the  various  properties  which 
it  has, — the  various  things  which  it  can  do. 

Its  Magnetic  Effects. 

96.  In  the  first  place,  it  can  deflect  the  magnetic  needle. 
For  instance,  let  a  compass  needle  be  swung  freely,  and 
let  a  current  of  electricity  circulate  along  a  wire  placed 
near  this  needle,  and  in  the  direction  of  its  length,  then 
the  direction  in  which  the  needle  points  will  be  imme- 
diately altered.  This  direction  will  now  depend  upon  that 
of  the  current,  conveyed  by  the  wire,  and  the  needle  will 
endeavour  to  place  itself  at  right  angles  to  this  wire. 

In  order  to  remember  the  connection  between  the 
direction  of  the  current  and  that  of  the  magnet,  imagine 
your  body  to  form  part  of  the  positive  current,  which  may 
be  supposed  to  enter  in  at  your  head,  and  go  out  at  your 
feet ;  also  imagine  that  your  face  is  turned  towards  the 
magnet.  In  this  case,  the  pole  of  the  magnet,  which 
points  to  the  north,  will  always  be  deflected  by  the  cur- 
rent towards  your  right  hand.  The  strength  of  a  current 
may  be  measured  by  the  amount  of  the  deflection  it  pro- 
duces upon  a  magnetic  needle,  and  the  instrument  by  which 
this  measurement  is  made  is  called  a  galvanometer. 


THE  FORCES  AND  ENERGIES  OF  NATURE. 


73 


97.  In  the  next  place,  the  current  is  able,  not  merely 
10  deflect  a  magnet,  but  also  to  render  soft  iron  magnetic. 
Let  us  take,  for  instance,  the  wire  =r:^-  "  ^^^ 
connected  with  the  one  pole  of  the 
battery,  and  cover  it  with  thread,  in 
order  to  insulate  it,  and  let  us  wrap 
this  wire  round  a  cylinder  of  soft 
iron,  as  in  Fig.  9.  If  we  now 
make  a  communication  between  the 
other  extremity  of  the  wire,  and 
the  other  pole  of  the  battery,  so  as 
to  make  the  current  pass,  it  will  be 
found  that  our  cylinder  of  soft  iron 
has  become  a  powerful  magnet,  and  that  if  an  iron 
keeper  be  attached  to  it  as  in  the  figure,  the  keeper 
wiU  be  able  to  sustain  a  very  great  weight. 


Fig.  9. 


Its  Heating  Effect, 

98.  The  electric  current  has  likeivise  the  property  oj 
heating  a  wire  through  which  it  passes.  To  prove  this, 
let  us  connect  the  two  poles  of  a  battery  by  means  of  a 
fine  platinum  wire,  when  it  will  be  found  that  the  wire 
will,  in  a  few  seconds,  become  heated  to  redness.  In 
point  of  fact,  the  current  will  heat  a  thick  wire,  but  not 
BO  much  as  a  thin  one,  for  we  may  suppose  it  to  rush  with 
great  violence  through  tlie  limited  section  of  the  thin 
wu^e,  producing  in  its  passage  great  heat. 


74  THE  CONSERVATION   OF  ENERGY. 

Its  Chemical  Effect 

99.  Besides  its  magnetic  and  heating  effects,  the  current 
has  also  the  poiver  of  decortiposing  compound  substances, 
under  certain  conditions.  Suppose,  for  instance,  that  the 
poles  of  a  battery,  instead  of  oeing  brought  together,  ai"e 
plunged  into  a  vessel  of  water,  decomposition  will  at  once 
begin,  and  small  bubbles  of  oxygen  will  rise  from  the 
positive  i^ole,  while  small  bubbles  of  hydrogen  will  make 
their  appearance  at  the  negative.  If  the  two  gases  are 
collected  together  in  a  vessel,  they  may  be  exploded,  and 
if  collected  separately,  it  may  be  proved  by  the  ordinary 
tests,  that  the  one  is  oxygen  and  the  other  hydrogen. 

Attraction  and  Repulsion  of  Currents. 

100.  We  have  now  described  very  shortly  the  magnetic, 
the  heating,  and  the  chemical  eftects  of  currents ;  it 
remains  for  us  to  describe  the  effects  of  currents  upon 
one  another. 

In  the  first  place,  suppose  that  we  have  two  wires 
which  are  parallel  to  one  another,  and  carry  cuiTcnts 
going  in  the  same  direction ;  and  let  us  further  suppose 
that  these  wires  are  capable  of  moving,  then  it  is  found 
that  they  will  attract  one  another.  If,  however,  the 
wires,  although  parallel,  convey  currents  going  in  opposite 
directions,  they  will  then  repel  one  another.  A  good  way 
of  showing  this  experimentally  is  to  cause  two  circular 
currents  to  float  on  water.     If  these  currents  both  go 


THE  FORCES  AND  ENERGIES  OF  NATURE.      75 

either  in  the  same  direction  as  the  hands  of  a  watch, 
or  in  the  opposite  direction,  then  the  two  will  attract 
one  another;  but  if  the  one  goes  in  the  one  direction, 
and  the  other  in  the  other,  they  will  then  i-epel  one 
another. 

Attraction  and  Repulsion  of  Magnets. 

101.  Ampere,  who  discovered  this  property  of  currents, 
has  likewise  shown  us  that  in  very  many  respects  a 
magnet  may  be  hkened  to  a  collection  of  circular  currents 
all  parallel  to  one  another,  their  direction  being  such  that, 
if  you  look  towards  the  north  pole  of  a  freely  suspended 
cylindrical  magnet  facing  it,  the  positive  current  will 
descend  on  the  east  or  left-hand  side,  and  ascend  on  the 
west  or  right-hand  side.  If  we  adopt  this  method  of 
viewing  magnets,  we  can  easily  account  for  the  attraction 
between  the  unlike  and  the  repulsion  between  the  like 
poles  of  a  magnet,  for  when  unlike  poles  are  placed 
near  each  other,  the  circular  currents  which  face  each 
other  are  then  all  going  in  the  same  direction,  and  the 
two  will,  therefore,  attract  one  another,  but  if  like  poles 
are  placed  in  this  position,  the  currents  that  -face  each 
other  are  going  in  opposite  directions,  and  the  poles  will, 
therefore,  repel  one  another. 

Induction  of  Currents. 

102.  Before  closing  this  short  sketch  of  electrical 
phenomena,   we  must  allude  to   the  inductive  effect  of 


re 


THE   CONSERVATION   OF   ENERGY. 


currents  upon  each  other.     Let  us  suppose  (Fig.  10)  that 

we  have  two  circular 
coils  of  wire,  covered 
with  thread,  and  placed 
near  each  other.  Let 
both  the  extremities  of 
the  right-hand  coil  be 
connected  with  the  poles 
of  a  battery,  so  as  to 
make  a  current  of  elec- 
tricity circulate  round 
the  coil.  On  the  other 
hand,  let  the  left-hand 
coil  be  connected  with 
a  galvanometer,  thus 
enabling  us  to  detect 
the  smallest  current  of 
electricity  which  may 
pass  through  this  coil. 
Now,  it  is  found  that 
when  we  first  connect 
the  )-ight-hand  coil,  so 
as  to  pass  the  battery 
current  through  it,  a 
momentary  current  will 
pass  through  the  left 
liand  coil,  and  will  de- 
llect  the  needle    of   the 


THE   FORCES  AND   ENERGIES   OF  NATURE.  77 

galvanometer,  but  this  current  will  go  in  an  opposite 
direction  to  that  which  circulates  round  the  right-hand 
coil. 

103.  Again,  as  long  as  the  current  continues  to  flow 
through  the  right-hand  coil  there  wiU  be  no  current 
through  the  other,  but  at  the  moment  of  breaking  the 
contact  between  the  right-hand  coil  and  the  battery  there 
will  again  be  a  momentary  current  in  the  left-hand  coil, 
but  this  time  in  the  same  direction  as  that  of  the  right- 
hand  coil,  instead  of  being,  as  before,  in  the  opposite 
direction.  In  other  words,  when  contact  is  'made  in  the 
right-hand  coil,  there  is  a  momentary  current  in  the  left- 
hand  coil,  but  in  an  opposite  direction  to  that  in  the  right, 
while,  when  contact  is  broken  in  the  right-hand  coil,  there 
is  a  momentary  current  in  the  left-hand  coil  in  the  same 
direction  as  that  in  the  right. 

104?.  In  order  to  exemplify  this  induction  of  currents, 
it  is  not  even  necessary  to  make  and  break  the  current 
in  the  right-hand  coil,  for  we  may  keep  it  constantly  going 
and  arrange  so  as  to  make  the  right-hand  coil  (always 
retaining  its  connection  with  the  battery)  alternately 
approach  and  recede  from  the  other ;  when  it  approaches 
the  other,  the  effect  produced  will  be  the  same  as  when 
the  contact  was  made  in  the  above  experiment — tliat  is 
to  say,  we  shall  have  an  induced  current  in  an  opposite 
direction  to  that  of  the  primary,  while,  when  it  recedes 
from  the  other,  we  shall  have  a  current  in  the  same  direc- 
tion as  that  of  the  primary. 


78  THE  CONSERVATION   OF  ENERGY. 

105.  Thus  we  see  that  whether  we  keep  both  coils 
stationary,  and  vuddenly  produce  a  current  in  the  right- 
hand  coil,  or  whether,  keeping  this  current  constantly 
going,  we  suddenly  bring  it  near  the  other  coil,  the 
inductive  effect  will  be  precisely  the  same,  for  in  both 
cases  the  left-hand  coil  is  suddenly  brought  into  the 
presence  of  a  current.  And  again,  it  is  the  same,  whether 
we  suddenly  break  the  right-hand  current,  or  suddenly 
remove  it  from  the  left-hand  ■  coil,  for  in  both  cases 
this  coil  is  virtually  removed  from  the  presence  of  a 
current. 

List  of  Energies. 

106.  We  are  now  in  a  position  to  enumerate  the  various 
kinds  of  energy  which  occur  in  nature  ;  but,  before  doing 
so,  we  must  warn  our  readers  that  this  enumeration  has 
nothing  absolute  or  complete  about  it,  representing,  as  it 
does,  not  so  much  the  present  state  of  our  knowledge  as 
of  our  want  of  knowledge,  or  rather  profound  ignorance, 
of  the  ultimate  constitution  of  matter.  It  is,  in  truth, 
orJy  a  convenient-  classification,  and  nothing  more. 

107.  To  begin,  then,  with  visible  energy.  We  have 
fi  rst  of  all — 

Energy  of  Visible  Motion. 

^A.)  Visible  energy  of  actual  motion — in  the  planets, 
in  meteors,  in  the  cannon  ball,  in  the  storm,  in 
the  running  stream,  and  in  other  instances  of 


THE  FORCES  AND  ENERGIES  OF  NATURE.      79 

bodies  in  actual  visible  motion,  too  numerous  to 
be  mentioned, 

Visihle  Energy  of  Position. 
(B.)  We  have  also  visible  energy  of  position — in  a  stone 
on  the  top  of  a  cliff,  in  a  head  of  water,  in  a  rain 
cloud,  in  a  cross-bow  bent,  in  a  clock  or  watch 
wound  up,  and  in  various  other  instances. 

108.  Then  we  have,  besides,  several  cases  in  which 
there  is  an  alternation  between  (A)  and  (B). 

A  pendulum,  for  instance,  when  at  its  lowest  point,  has 
only  the  energy  (A),  or  that  of  actual  motion,  in  virtue  of 
which  it  ascends  a  certain  distance  against  the  force  of 
gravity.  When,  however,  it  has  completed  its  ascent,  its 
energy  is  then  of  the  variety  (B),  being  due  to  position, 
and  not  to  actual  motion;  and  so  on  it  continues  to 
oscillate,  alternately  changing  the  nature  of  its  energy 
from  (A)  to  (B),  and  from  (B)  back  again  to  (A). 

109.  A  vibrating  body  is  another  instance  of  this  alter- 
nation. Each  particle  of  such  a  body  may  be  compared  to 
an  exceedingly  small  pendulum  oscillating  backwards 
and  forwards,  only  very  much  quicker  than  an  ordinary 
pendulum ;  and  just  as  the  ordinary  pendulum  in  passing 
its  point  of  rest  has  its  energy  all  of  one  kind,  while  in 
passing  its  upper  point  it  has  it  all  of  another,  so  when 
a  vibrating  particle  is  passing  its  point  of  rest,  its  energy 
is  all  of  the  variety  (A),  and  when  it  has  reached  its 
extreme  displacement,  it  is  all  of  the  variety  (B). 


so  THE  CONSERVATION  OF  ENEfiGY. 

Heat  Motion, 

110.  (C.)  Coming  now  to  molecular  or  invisible  energy, 

we  have,  in  the  first  place,  that  motion  of  the 
molecules  of  bodies  which  we  term  heat.  A 
better  term  would  be  absorbed  heat,  to  distin- 
guish it  from  radiant  heat,  which  is  a  very 
different  thing.  That  peculiar  motion  which  is 
imparted  by  heat  when  absorbed  into  a  body  is, 
therefore,  one  variety  of  molecular  energy. 

Molecular  Separation. 

(D.)  Analagous  to  this  is  that  effect  of  heat  which 
represents  position  rather  than  actual  motion. 
For  part  of  the  energy  of  absorbed  heat  is  spent 
in  pulling  asunder  the  molecules  of  the  body 
under  the  attractive  force  which  binds  them 
together  (Art.  73),  and  thus  a  store  of  energy  of 
position  is  laid  up,  which  disappears  again  after 
the  body  is  cooled. 

Atomic  or  Chemical  Separation. 

111.  (E.)  The  two  previous  varieties  of  energy  may  be 

viewed  as  associated  with  molecules  rather  than 
with  atoms,  and  with  the  force  of  cohesion 
rather  than  with  that  of  chemical  affinity. 
Proceeding  now  to  atomic  force,  we  have 
a  species   of   energy   of    position   due    to   the 


THE  FORCES  AND  ENERGIES  OF  NATURE.      81 

separation  of  different  atoms  under  the  strong 
chemical  attraction  they  have  for  one  another. 
Thus,  when  we  possess  coal  or  carbon  and  also 
oxygen  in  a  state  of  separation  from  one 
another,  we  are  in  possession  of  a  source  of 
energy  which  may  be  called  that  of  chemical 
separation. 

Electrical  Separation. 

112  (F.)  The  attraction   which   heterogeneous  atoms 

possess  for  one  another,  sometimes,  however, 
gives  rise  to  a  species  of  energy  which  mani- 
fests itself  in  a  very  peculiar  form,  and 
appears  as  electrical  separation,  which  is  thus 
anotlier  form  of  energy  of  position. 

Electricity  in  Motion. 

113  (G.)  But  we  have  another  species  of  energy  con- 

nected with  electricity,  for  we  have  that  due  to 
electricity  in  motion,  or  in  other  words,  an 
electric  current  which  probably  represents  some 
form  of  actual  motioru 

Radicmt  Energy. 

114  (HJ  It  is  well  kno^vn  that  there  is  no  ordinary 

matter,  or  at  least  hardly  any,  between  tlie  sun 
and  the  earth,  and  yet  we  have  a  kind  of  energy 


82  THE   CONSERVATION   OF  ENERGY. 

which  we  may  call  radiant  energy,  which  j)ro- 
ceeds  to  us  from  the  sun,  and  proceeds  also  with 
a  definite,  though  very  great  velocity,  taking 
about  eight  minutes  to  perform  its  journey. 
Now,  this  radiant  energy  is  known  to  consist  of 
the  vibrations  of  an  elastic  medium  pervading 
all  space,  which  is  called  ether,  or  the  etherial 
friedimn.  Inasmuch,  therefore,  as  it  consists 
of  vibrations,  it  partakes  of  the  character  of 
pendulum  motion,  that  is  to  say,  the  eneigy  of 
any  ethereal  particle  is  alternately  that  of 
position  and  that  of  actual  motion. 

Laiv  of  Conservation. 

115.  Having  thus  endeavoured,  provisionally  at  least, 
to  catalogue  our  various  energies,  we  are  in  a  position 
to  state  more  definitely  what  is  meant  by  the  conserva- 
tion of  energy.  For  this  purpose,  let  us  take  the  universe 
as  a  whole,  or,  if  this  be  too  large,  let  us  conceive,  if 
possible,  a  small  portion  of  it  to  be  isolated  from  the  rest, 
as  far  as  force  or  energy  is  concerned,  forming  a  sort  of 
microcosm,  to  which  we  may  conveniently  direct  our 
attention. 

This  portion,  then,  neither  parts  with  any  of  its 
energy  to  the  universe  beyond,  nor  receives  any  from  it. 
Such  an  isolation  is,  of  course,  unnatural  and  impossible, 
but  it  is  conceivable,  and  will,  at  least,  tend  to  concentrate 
our  thoughta    Now,  whether  we  regard  the  great  universe, 


STATEMENT  OF  THE  LAW  OF  CONSERVATION.    83 

or  this  small  microcosm,  the  principle  of  the  conservation 
of  energy  asserts  that  the  sum  of  all  the  various  energies 
is  a  constant  quantity,  that  is  to  say,  adopting  the  lan- 
guage of  Algebra — • 

(A)  +  (B)  +  (C)  +  (D)  +  (E)  +  (F)  +  (G)  +  (H)  =  a 
constant  quantity. 

lie.  This  does  not  mean,  of  course,  that  (A)  is  constant 
in  itself,  or  any  other  of  the  left-hand  members  of  this 
equation,  for,  in  truth,  they  are  always  changing  about 
into  each  other — now,  some  visible  energy  being  changed 
into  heat  or  electricity  ;  and,  anon,  some  heat  or  electricity 
being  changed  back  again  into  visible  energy — but  it 
only  means  that  the  sum  of  all  the  energies  taken  together 
is  constant.  We  have,  in  fact,  in  the  left  hand,  eight 
variable  quantities,  and  we  only  assert  that  their  sum  is 
constant,  not  by  any  means  that  they  are  constant  them- 
selves. 

117.  Now,  what  evidence  have  we  for  this  assertion  ? 
It  may  be,  replied  that  we  have  the  strongest  possible 
evidence  which  the  nature  of  the  case  admits  of.  The 
assertion  is,  in  truth,  a  peculiar  one — peculiar  in  its  mag- 
nitude, in  its  universality,  in  the  subtle  nature  of  the 
agents  with  which  it  deals.  If  true,  its  truth  certainly 
cannot  be  proved  after  the  manner  in  which  we  prove  a 
proposition  in  Euclid.  Nor  does  it  even  admit  of  a  proof 
so  rigid  as  that  of  the  somewhat  analogous  principle  ot 
the  conservation  of  matter,  for  in   chemistry   we   may 


8i  THE   CONSERVATION  OF  ENERGY. 

confine  the  products  of  our  chemical  comlbination  so 
completely  as  to  prove,  beyond  a  doubt,  that  no  heavy 
matter  passes  out  of  existence  that — when  coal,  for  in- 
stance, burns  in  oxygen  gas — what  we  have  is  merely  a 
change  of  condition.  But  we  cannot  so  easily  prove  that 
no  energy  is  destroyed  in  this  combination,  and  that  the 
only  result  is  a  change  from  the  energy  of  chemical 
separation  into  that  of  absorbed  heat,  for  during  the 
process  it  is  impossible  to  isolate  the  energy — do  what 
we  may,  some  of  it  wiU  escape  into  the  room  in  which  we 
perform  the  experiment ;  some  of  it  will,  no  doubt,  escape 
through  the  window,  while  a  little  will  leave  the  earth 
altogether,  and  go  out  into  space.  All  that  we  can  do 
in  such  a  case  is  to  estimate,  as  completely  as  possible, 
how  much  energy  has  gone  away,  since  we  cannot  possibly 
prevent  its  going.  But  this  is  an  operation  involving 
great  acquaintance  with  the  laws  of  energy,  and  very 
great  exactness  of  observation :  in  fine,  our  readers  will 
at  once  perceive  that  it  is  much  more  difficult  to  prove 
the  truth  of  the  conservation  of  energy  than  that  of  the 
conservation  of  matter. 

118.  But  if  it  be  difficult  to  prove  our  principle  in 
the  most  rigorous  manner,  we  are  yet  able  to  give  the 
strongest  possible  indirect  evidence  of  its  truth. 

Our  readers  are  no  doubt  familiar  with  a  method 
which  Euclid  frequently  adopts  in  proving  his  proposi- 
tions. Starting  with  the  supposition  that  they  are  not 
true,  and  reasoning  upon  this  hypothesis,  he  comes  to 


STATEMENT  OF  THE  LAW  OF  CONSEKVATION.    85 

an  absurd  conclusion — hence  he  concludes  that  they  are 
true.  Now,  we  may  adopt  a  method  somewhat  similar 
with  reo-ard  to  our  principle,  only  instead  of  sup- 
posing it  untrue,  let  us  su2)pose  it  true.  It  may  then 
be  shown  that,  if  it  be  true,  under  certain  test  conditions 
we  ought  to  obtain  certain  results — for  instance,  if  we 
increase  the  pressure,  we  ought  to  lower  the  freezing 
point  of  water.  Well,  we  make  the  experiment,  and 
find  that,  in  point  of  fact,  the  freezing  point  of  water  is 
lowered  by  increasing  the  pressure,  and  we  have  thus 
derived  an  argument  in  favour  of  the  conservation  of 
energy. 

119.  Or  again,  if  the  laws  of  energy  are  true,  it  may 
be  shown  that,  whenever  a  substance  contracts  when 
heated,  it  will  become  colder  instead  of  hotter  by  com- 
pression. Now,  we  know  that  ice-cold  water,  or  water 
just  a  little  above  its  freezing  point,  contracts  instead 
of  expanding  up  to  4°  C. ;  and  Sir  William  Thomson  has 
found,  by  experiment,  that  water  at  this  temperature  is 
cooled  instead  of  heated  by  sudden  compression.  India- 
rubber  is  another  instance  of  this  relation  between  these 
two  properties,  for  if  we  stretch  a  string  of  india-rubber  ii 
gets  hotter  instead  of  colder,  that  is  to  say,  its  tempera- 
ture rises  by  extension,  and  gets  lower  by  contraction  ; 
and  again,  if  we  heat  the  string,  we  find  that  it  contracts 
in  length  instead  of  expanding  like  other  substances  as 
its  temperature  increases. 

120.  Numberless   instances    occur   in    which    -we   ai'O 


86  THE  CONSERVATION   OF  ENERGY. 

enabled  to  predict  what  will  happen  by  assuming  the 
truth  of  the  laws  of  energy ;  in  other  words,  these  laws 
are  proved  to  be  true  in  all  cases  where  we  can  put  them 
to  the  test  of  rigorous  experiment,  and  probably  we  can 
have  no  better  proof  than  this  of  the  truth  of  such  a 
principle.  We  shall  therefore  proceed  upon  the  assumption 
that  the  conservation  of  energy  holds  true  in  all  cases, 
and  give  our  readers  a  list  of  the  various  transmutations 
of  this  subtle  agent  as  it  goes  backwards  and  forwards 
from  one  abode  to  another,  making,  meanwhile,  sundry 
remarks  that  may  tend,  we  trust,  to  convince  our  readers 
jf  the  truth  of  our  assumptioiL 


TEAKbMUTATiONb   Oi    ENKIiG^.  87 


CHAPTER  IV. 

TRANSMUTATIONS  OF  ENERGY. 
Energy  of  Visible  Motio7i. 

121  Let  us  begin  our  list  of  transmutations  witli  the 
or  ergy  of  visible  motion.  This  is  changed  int(j  energi/ 
of  2Josition  when  a  stone  is  projected  upwards  above  the 
earth,  or,  to  take  a  case  precisely  similar,  when  a  planet 
or  a  comet  goes  from  perihelion,  or  its  position  nearest  the 
sun,  to  aphelion,  or  its  position  furthest  from  the  sun.  We 
thus  see  why  a  heavenly  body  should  move  fastest  at 
perihelion,  and  slowest  at  aphelion.  If,  however,  a 
planet  were  to  move  round  the  sun  in  an  orbit  exactly 
circular,  its  velocity  would  be  the  same  at  all  the  various 
points  of  this  orbit,  because  there  would  be  no  change 
in  its  distance  from  the  centre  of  attraction,  and  there- 
fore no  transmutation  of  energy. 

122.  We  have  already  (Arts.  108,  109)  said  that  the 
energy  in  an  oscillating  or  vibrating  body  is  alternately 
that  of  actual  motion,  and  that  of  position.  In  this 
respect,  therefore,  a  pendulum  is  similar  to  a  comet  or 
heavenly  body  with  an  elliptical  orbit.     Nevertheless  the 

5 


t^8  THE   CONSERVATION    OF   ENERCIV. 

change  uf  energy  is  generally  more  complete  in  a  pendalimi 
or  vibrating  body  than  it  is  in  a  heavenly  body ;  for  in  a 
pendulum,  when  at  its  lowest  point,  the  energy  is  entirely 
that  of  actual  motion,  while  at  its  upper  point  it  is 
entirely  that  of  position.  Now,  in  a  heavenly  body  we 
have  only  a  lessening,  but  not  an  entire  destruction,  of 
the  velocity,  as  the  body  passes  from  perihelion  to 
aphelion — that  is  to  say,  we  have  only  a  partial  conver- 
sion of  the  one  kind  of  energy  into  the  other. 

123.  Let  us  next  consider  the  change  of  actual  visible 
energy  into  absorbed  heat  This  takes  place  in  all  cases 
of  friction,  percussion,  and  resistance.  In  friction,  for 
instance,  we  have  the  conversion  of  work  or  energy  into 
heat,  which  is  here  produced  through  the  rubbing  of  surfaces 
against  each  other ;  and  Davy  has  shown  that  two  pieces 
of  ice,  both  colder  than  the  freezing  point,  may  be  melted 
by  friction.  In  percussion,  again,  we  have  the  energy 
of  the  blow  converted  into  heat  ;  while,  in  the  case  of  a 
meteor  or  cannon  ball  passing  through  the  air  with  great 
v^elocity,  we  have  the  loss  of  energy  of  the  meteor  or 
cannon  ball  through  its  contact  with  the  air,  and  at  the 
same  time  the  production  of  heat  on  account  of  this 
resistance. 

The  resistance  need  not  be  atmospheric,  for  we  may 
yet  the  cannon  ball  to  pierce  through  wooden  planks  or 
through  sand,  and  there  will  equally  be  a  production  of 
heat  on  account  of  the  resistance  offered  by  the  wooden 
planks  or  by  the  sand  to  the  motion  of  the  ball.     We 


TRANSMUTATIONS   OF   ENERGY.  89 

may  even  generalize  still  further,  and  assert  that  when- 
ever the  visible  momentum  of  a  body  is  transferred  to  a 
larger  mass,  there  is  at  the  same  time  the  conversion  of 
visible  energy  into  heat. 

124  A  little  explanation  will  be  required  to  make  this 
point  clear. 

The  third  law  of  motion  tells  us  that  action  and  re- 
action are  equal  and  opposite,  so  that  when  two  bodies 
come  into  collision  the  forces  at  work  generate  equal  and 
opposite  quantities  of  momentum.  We  shall  best  see 
the  meaning  of  this  law  by  a  numerical  example,  bear- 
ing in  mind  that  momentum  means  the  product  of  mass 
into  velocity. 

For  instance,  let  us  suppose  that  an  inelastic  body  of 
mass  10  and  velocity  20  strikes  directly  another  inelastic 
body  of  mass  15  and  velocity  15,  the  direction  of  both 
motions  being  the  same. 

Now,  it  is  well  known  that  the  united  mass  will,  after 
impact,  be  moving  with  the  velocity  17.  What,  then,  has 
been  the  influence  of  the  forces  developed  by  collision  ? 
The  body  of  greater  velocity  had  before  impact  a 
momentum  10  x  20  =  200,  while  its  momentum  after 
impact  is  only  10x17  =  170 ;  it  has  therefore  suffered 
a  loss  of  30  units  as  regards  momentum,  or  we  may  con- 
sider that  a  momentum  of  30  units  has  been  impressed 
upon  it  in  an  opposite  direction  to  its  previous  motion. 

On  the  other  hand,  the  body  of  smaller  velocity  had 
bt'fure   impact  a  momentum   15  x  15  =  225,  while  after 


90  THE   CONSERVATION   OF  ENEilGV. 

impact  it  has  15  X  17  =  255  units,  so  that  its  momentum 
has  been  increased  by  30  units  in  its  previous  direction. 

The  force  of  impact  has  therefore  generated  30  units 
of  momentum  in  two  opposite  directions,  so  that,  taking 
account  of  direction,  the  momentum  of  the  system  is 
the  same  before  and  after  impact ;  for  before  impact  we 
had  a  momentum  of  10  x  20  +  15  x  15  =  425,  while  after 
it  we  have  the  united  mass  25  moving  with  the  velocity 
17,  giving  the  momentum  425  as  before. 

125.  But  while  the  momentum  is  the  same  before  and 
after  impact,  the  visible  energy  of  the  moving  mass  is 
undoubtedly  less  after  impact  than  before  it.  To  see 
this  we  have  only  to  turn  to  the  expression  of  Art.  28, 
from  which    we    find    that    the    energy   before    impact 

was  as  follows : — Energy  in  kilogrammetres  =  TqTp  ^^ 

10^x20" +  15  xl5^  ^  ^^Q  nearly ;  while  that  after  impact 
19  ■  6 

25  X  172       368  nearly. 


19-6 

120.  The  loss  of  energy  will  be  still  more  manifest  if  we 
suppose  an  inelastic  body  in  motion  to  strike  against  a 
similar  body  at  rest.  Thus  if  we  have  a  body  of  mass 
20  and  velocity  20  striking  against  one  of  equal  mass, 
but  at  rest,  the  velocity  of  the  double  mass  after  impact 
will  obviously  be  only  10 ;  but,  as  regards  energy,  that 

.„  ,      20  X  20  2       8000      ,  .,    ,,    ,     p. 
before  impact  will  be  — j^vq-  =  jgTg  while  that  attcv 


TRANSMUTATIONS   OF  ENERGY.  91 

.N«^.«f     -n  I      40x10-      4000  ,     i    li- .i     -• 

impact  will  be    -— -  =   -— -.    or  only  half  the  former. 

19-6         19-G  "^ 

127.  Tlm,f  there  is  in  all  such  cases  an  apparent  loss  of 
visible  ene\^/,  while  at  the  same  time  there  is  the  pi'o- 
duction  of  ho'it  on  account  of  the  bloAV  which  takes 
place.  If,  however,  the  substances  that  come  together  be 
perfectly  elastic  (which  no  substance  is),  the  visible  energy 
after  imp/'.it  will  be  the  same  as  that  before,  and  in  this 
case  thciv  will  be  no  conversion  into  heat.  This,  however, 
is  an  extreme  supposition,  and  inasmuch  as  no  substance 
is  perfectly  elastic,  we  have  in  all  cases  of  collision  a 
greater  or  less  conversion  of  visible  motion  into  heat. 

128.  We  have  spoken  (Art.  122)  about  the  change  of 
energy  in  an  oscillating  or  vibrating  body,  as  if  it  were 
entirely  one  of  actual  energy  into  energy  of  position, 
and  the  reverse. 

But  even  here,  in  each  oscillation  or  vibration,  there  is 
a  greater  or  less  conversion  of  visible  energy  into  heat. 
Let  us,  for  instance,  take  a  pendulum,  and,  in  order  to 
ma,ke  the  circumstances  as  favourable  as  possible,  let  it 
swing  on  a  knife  edge,  and  in  vacuo ;  in  this  case  there 
will  be  a  slight  but  constant  friction  of  the  knife  edo-e 
against  the  plane  on  which  it  rests,  and  though  the 
pendulum  may  continue  to  swing  for  hours,  yet  it  will 
ultimately  come  to  rest. 

And,  again,  it  is  impossible  to  make  a  vacuum  so  perfect 
that  there  is  absolutely  no  air  surrounding  the  pendulum, 
so  that  part  of  the  motion  of  the  pendulum  will  always 


D2  THE  conservaticTn  of  energy. 

be  carried    off   by  the   residual   air  of   the   vacuum  iu 
which  it  swings. 

129.  Now,  something  similar  happens  in  that  vibratory 
motion  which  constitutes  sound.  Thus,  when  a  bell  is  in 
vibration,  part  of  the  energy  of  the  vibration  is  carried 
off  by  the  surrounding  air,  and  it  is  in  virtue  of  this  that 
we  hear  the  sound  of  the  bell ;  but,  even  if  there  were  no 
au',  the  bell  would  not  go  on  vibrating  for  ever.  For 
there  is  in  all  bodies  a  greater  or  less  amount  of  internal 
viscosity,  a  property  which  prevents  perfect  freedom  of 
vibration,  and  which  ultimately  converts  vibrations  into 
heat. 

A  vibrating  bell  is  thus  very  much  in  the  same  position 
as  an  oscillating  pendulum,  for  in  both  part  of  the  energy 
is  given  off  to  the  air,  and  in  both  there  is  unavoidable 
friction — in  the  one  taking  the  shape  of  internal  vis- 
cosity, and  in  the  other  that  of  friction  of  the  knife  edge 
against  the  plane  on  which  it  rests. 

130.  In  both  these  cases,  too,  that  jDortion  of  the  energy 
w^hich  goes  into  the  air  takes  ultimately  the  shaj^e  of 
heat.  The  oscillating  pendulum  communicates  a  motion 
to  the  air,  and  this  motion  ultimately  heats  the  air.  The 
vibrating  bell,  or  musical  instrument,  in  like  manner  com- 
municates part  of  its  energ}'-  to  the  air.  This  communi- 
cated energy  first  of  all  moves  through  the  air  with  the 
well-known  velocity  of  sound,  but  during  its  progress  it, 
too,  no  doubt  becomes  partly  converted  into  heat. 
Ultimately,  it  is  transmitted  by  the  air  to  other  bodies, 


THANSMUTATIOXS   OF   ENERGY.  93 

aud  by  means  of  tlicir  internal  viscosity  is  sooner  or 
later  converted  into  heat.  Thus  we  see  that  heat  is  the 
form  of  energy,  into  which  all  visible  terrestrial  motion, 
whether  it  be  rectilinear,  or  oscillatory,  or  vibratory,  is 
ultimately  changed. 

131.  In  the  case  of  a  body  in  visible  rectilinear  motion 
on  the  earth's  surface,  this  change  takes  place  very  soon — 
if  the  motion  be  rotatory,  such  as  that  of  a  heavy  re- 
volving top,  it  may,  perhaps,  continue  longer  before  it  i« 
ultimately  stopped,  by  means  of  the  surrounding  air,  and 
by  friction  of  the  pivot;  if  it  be  oscillatory,  as  in  the 
pendulum,  or  vibratory,  as  in  a  musical  instrument,  wo 
have  seen  that  the  air  and  internal  friction  are  at  work, 
in  one  shape  or  another,  to  carry  it  off,  and  will  ultimately 
succeed  in  converting  it  into  heat. 

132.  But,  it  may  be  said,  why  consider  a  body  moving 
on  the  earth's  surface  ?  why  not  consider  the  motion 
of  the  earth  itself?  Will  this  also  ultimately  take 
the  shape  of  heat  ? 

No  doubt  it  is  more  difficult  to  trace  the  conversion 
in  such  a  case,  inasmuch  as  it  is  not  proceeding  at  a 
sensible  rate  before  our  eyes.  In  other  words,  the 
very  conditions  that  make  the  earth  habitable,  and  a 
fit  abode  for  intelligent  beings  like  ourselves,  are  those 
which  unfit  us  to  perceive  this  conversion  of  energy 
in  the  case  of  the  earth.  Yet  we  are  not  without 
indications  that  it  is  actually  taking  place.  For  the 
purpose  of  exhibiting  these,  we  may  divide  the  earth's 


34  THE  CONSEIIVATIOX   OF  ENERGY, 

motion  into  t^o — a  motion  of  rotation,  and  one  of  revo- 
lution, 

133.  Now,  with  regard  to  the  earth's  rotation,  the  con- 
version of  the  visible  energy  of  this  motion  into  heat  is 
already  well  recognized.  To  understand  this  we  have 
only  to  study  the  nature  of  the  moon's  action  upon  the 
fluid  portions  of  our  globe.  In  the  following  diagram 
(Fig.  11)  we  have  an  exaggerated  representation  of  this, 
by  which  we  see  that  the  spherical  earth  iii  converted 


Moon 

Fig.  11. 

into  an  elongated  oval,  of  which  one  extremity  always 
points  to  the  moon.  The  solid  body  of  the  earth  itselt 
revolves  as  usual,  but,  nevertheless,  this  fluid  protuber- 
ance remains  always  pointing  towards  the  moon,  as  we 
see  in  the  figure,  and  hence  the  earth  rubs  against  the 
protuberance  as  it  revolves.  The  friction  produced  by 
this  action  tends  evidently  to  lessen  the  rotatory  energy 
of  the  earth — in  other  words,  it  acts  like  a  break — and  we 
have,  just  as  by  a  break-wheel,  the  conversion  of  visible 
f-nergy  into  heat.  This  was  first  recognized  by  Mayer 
and  J.  Thomson. 

134.  But  while  there  can  be  no  doubt  about  the  fact  of 
Kuch  a  conversion  going  on,  the  only  question  is  regarding 


TRANSMUTATIONS   OF   ENERGY.  95 

its  rate  of  progress,  and  the  time  required  before  it  can 
cause  a  perceptible  impression  upon  the  rotative  energy 
of  the  earth. 

Now,  it  is  believed  by  astronomers  that  they  have 
detected  evidence  of  such  a  change,  for  our  knowledge  of 
the  motions  of  the  sun  and  moon  has  become  so  exact, 
that  not  only  can  we  carry  forward  our  calculations  so  as 
to  predict  an  eclipse,  but  also  carry  them  backwards,  and 
thus  fix  the  dates  and  even  the  very  details  of  the 
ancient  historical  eclipses. 

If,  however,  between  those  times  and  the  present,  the 
earth  has  lost  a  little  rotative  energy  on  account  of  this 
peculiar  action  of  the  moon,  then  it  is  evident  that  the 
calculated  circumstances  of  the  ancient  total  eclipse  will 
not  quite  agree  with  those  actually  recorded  ;  and  by 
a  comparison  of  this  nature  it  is  believed  that  we 
have  detected  a  very  slight  falling  off  in  the  rotative 
energy  of  our  earth.  If  we  carry  out  the  argument,  we 
shall  be  driven  to  the  conclusion  that  the  rotative  energy 
of  our  globe  will,  on  account  of  the  moon's  action,  alwajss 
get  less  and  less,  until  things  are  brought  into  such  a 
state  that  the  rotation  comes  to  be  performed  in  the  same 
time  as  the  revolution  of  the  moon,  so  that  then  the  same 
portion  of  the  terrestrial  surface  being  always  presented 
to  the  moon,  it  is  evident  that  there  will  be  no  effort 
made  by  the  solid  substance  of  the  earth,  to  glide  from 
under  the  fluid  protuberance,  and  there  will  in  conse- 
quence be  no  friction,  and  no  further  loss  of  energy. 


96  THE  CONSEEVATION  OF  ENEEGY. 

135.  If  the  fate  of  the  earth  be  ultimately  to  turn  the 
same  face  always  to  the  moon,  we  have  abundant  evidence 
that  this  very  fate  has  long  since  overtaken  the  moon 
herself  Indeed,  the  much  stronger  effect  of  our  earth 
upon  the  moon  has  produced  this  result,  probably,  even 
in  those  remote  periods  when  the  moon  was  chiefly  fluid  j 
and  it  is  a  fact  well  known,  not  merely  to  astronomers, 
but  to  all  of  us,  that  the  moon  nowadays  turns  always 
the  same  face  to  the  earth.*  No  doubt  this  fate  has  long 
since  overtaken  the  satellites  of  Jupiter,  Saturn,  and  the 
otlicr  large  planets  ;  and  there  are  independent  indications 
that,  at  least  in  the  case  of  Jupiter,  the  satellites  turn 
always  the  same  face  to  their  primary. 

136.  To  come  now  to  the  energy  of  revolution  of  the 
earth,  in  her  orbit  round  the  sun,  we  cannot  help  believ- 
ing that  there  is  a  material  medium  of  some  kind  between 
the  sun  and  the  earth ;  indeed,  the  undulatory  theory  of 
light  requires  this  belief  But  if  we  believe  in  such  a 
medium,  it  is  diiScult  to  imagine  that  its  presence  w.ill 
not  ultimately  diminish  the  motion  of  revolution  of  the 
earth  in  her  orbit ;  indeed,  there  is  a  strong  scientific 
probability,  if  not  an  absolute  certainty,  that  such  will  be 
the  case.  There  is  even  some  reason  to  think  that  the 
energy  of  a  comet  of  small  period,  called  Encke's  comet,  is 
gradually  being  stopped  from  this  cause ;  in  fine,  there  can 
be  hardly  any  doubt  that  the  cause  is  really  in  operation, 

*  This  explanation  was  first  given  by  Professors  Thomson  and  Tait  in 
their  Natural  Philosophy,  and  by  Dr.  Frankland  in  a  lecture  at  the  Eoyal 
Institution  of  London. 


TRAI^SMUTATIONS   OF  ENERGY.  97 

and  will  ultimately  affect  the  motions  of  the  planets  and 
oLher  heavenly  bodies,  even  although  its  rate  of  action 
may  be  so  slow  that  we  are  not  able  to  detect  it. 

We  may  perhaps  generalize  by  saying,  that  wherever 
in  the  universe  there  is  a  differential  motion,  that  is  to 
say,  a  motion  of  one  part  of  it  towards  or  from  another, 
then,  in  virtue  of  the  subtle  medium,  or  cement,  that  binds 
the  various  parts  of  the  universe  together,  this  motion  is 
not  unattended  by  something  like  friction,  in  virtue  of 
which  the  differential  motion  will  ultimately  disappear, 
while  the  loss  of  energy  caused  by  its  disappearance  will 
assume  the  form  of  heat.' 

137.  There  are,  indeed,  obscure  intimations  that  a  con- 
version of  this  kind  is  not  improbably  taking  place  in  the 
solar  system ;  for,  in  the  sun  himself,  we  have  the  matter 
near  the  equator,  by  virtue  of  the  rotation  of  our  lumi- 
nary, carried  alternately  towards  and  from  the  various 
planets.  Now,  it  would  seem  that  the  sun-spots,  or 
atmospheric  disturbances  of  the  sun,  affect  particularly 
his  equatorial  regions,  and  have  likewise  a  tendency  to 
attain  their  maximum  size  in  that  position,  which  is  as 
far  away  as  possible  from  tjie  influential  planets,  such  as 
Mercury  or  Venus ;  *  so  that  if  Venus,  for  instance, 
were  between  the  earth  and  the  sun,  there  would  be  few 
sun-spots  in  the  middle  of  the  sun's  disc,  because  that 
would  be  the  part  of  the  sun  nearest  Venus. 

*  See  De  La  Hue,  Stewart,  and  Loewy's  researches  on  Solar  Physicu. 


yS  THE  CONSERVATION   OF  ENERGY. 

But  if  the  j)lanets  influence  sun-spots^  the  action  is  no 
doubt  reciprocal,  and  we  have  much  reason  to  believe  that 
sun-spots  influence,  not  only  the  magnetism,  but  also  the 
meteorology  of  our  earth,  so  that  there  are  most  dis])lays 
of  the  Aurora  Borealis,  as  well  as  most  cyclones,  in  those 
years  when  there  are  most  sun-spots.  *  Is  it  not  then 
possible  that,  in  these  strange,  mysterious  phenomena, 
we  see  traces  of  the  machinery  by  means  of  which  the 
differential  motion  of  the  solar  system  is  gradually  being 
changed  into  heat  ? 

138.  We  have  thus  seen  that  visible  energy  of  actual 
motion  is  not  unfrequently  changed  into  visible  energy  of 
position,  and  that  it  is  also  very  often  transformed  into 
absorbed  heat.  We  have  now  to  state  that  it  may  like- 
wise be  transformed  into  electrical  sejmration.  Thus,  when 
an  ordinary  electrical  machine  is  in  action,  considerable 
labour  is  spent  in  turning  the  handle ;  it  is,  in  truth, 
harder  to  turn  than  if  no  electricity  were  being  produced — 
in  other  words,  part  of  the  energy  which  is  spent  upon 
the  machine  goes  to  the  production  of  electrical  separation. 
There  are  other  ways  of  generating  electricity  besides  the 
frictional  method.  If,  for  instance,  we  bring  an  insulated 
conducting  plate  near  the  prime  conductor  of  the  electrical 
machine,  yet  not  near  enough  to  cause  a  spark  to  pass, 
and  if  we  then  touch  the  insulated  plate,  we  shall  find  it, 
after  contact,  to  be  charged  with  an  electricity  the  oppo- 

*  See  the  Magnetic  Researches  of  Sir  E.  Suhine,  also  C.  Meldvum  on 
the  Periodicity  of  Ci^clones. 


TRANSMUTATIONS   OF   ENERGY.  99 

site   of  tliat  in  the  macliine ;   we  may   then  remove  it 
ami  make  use  of  this  electricity. 

It  retiuires  a  little  thought  to  see  what  labour  we  have 
spent  in  this  process.  We  must  bear  in  mind  that,  by 
touching  the  plate,  we  have  carried  off  the  electricity  of 
the  same  name  as  that  of  the  machine,  so  that,  after 
touching  the  insulated  plate  it  is  more  strongly  attracted 
to  the  conductor  than  it  was  before.  When  we  begin  to 
remove  it,  therefore,  it  will  cost  us  an  effort  to  do  so,  and 
the  mechanical  energy  which  we  spend  in  removing  it 
will  account  for  the  energy  of  electrical  separation  which 
we  then  obtain. 

139.  We  may  thus  make  use  of  a  small  nucleus  of 
electricity,  to  assist  us  in  procuring  an  unlimited  supply, 
for  in  the  above  process  the  electricity  of  the  prime  con- 
ductor remains  unaltered,  and  Ave  may  repeat  the 
operation  as  often  as  we  like,  and  gather  together  a  very 
large  quantity  of  electricity,  without  finally  altering  the 
electricity  of  the  prime  conductor,  but  not,  however, 
without  the  expenditure  of  an  equivalent  amount  of 
energy,  in  the  shape  of  actual  work. 

140.  While,  as  we  have  seen,  there  is  a  tendency  in  all 
motion  to  be  changed  into  heat,  there  is  one  instance 
where  it  is,  in  the  first  place  at  least,  changed  into  a  current 
of  eledricifi/.  We  allude  to  the  case  where  a  conducting 
substance  moves  in  the  presence  of  an  electric  current,  or 
of  a  magnet. 

In  Art.  104  we  found  that  if  one  coil  connected  with  a 


100  THE   CONSERVATION   OF   ENERGY. 

battery  were  quicldy  moved  into  the  presence  of  another 
coil  connected  with  a  galvanometer,  an  induced  current 
would  be  venerated  in  the  latter  coil,  and  would  affect 
the  galvanometer,  its  direction  being  the  reverse  of  that 
passing  in  the  other.  Now,  an  electric  current  implies 
energy,  and  we  may  therefore  conclude  that  some  other 
form  of  energy  must  be  spent,  or  disappear,  in  order  to 
produce  the  current  which  is  generated  in  the  coil 
attached  to  the  galvanometer. 

Again,  we  learn  from  Art.  100  that  two  currents  going 
in  opposite  directions  repel  one  another.  The  current 
generated  in  the  coil  attached  to  the  galvanometer  or 
secondary  current  will,  therefore,  repel  the  primary 
current,  which  is  moving  towards  it ;  this  repulsion  will 
either  cause  a  stoppage  of  motion,  or  render  necessary 
the  expenditure  of  energy,  in  order  to  keep  up  the 
motion  of  this  moving  coil.  We  thus  find  that  two 
phenomena  occur  simultaneously.  In  the  first  place, 
there  is  the  production  of  energy  in  the  secondary  coil, 
in  the  shape  of  a  current  opposite  in  direction  to 
that  of  the  primary  coil ;  in  the  next  case,  owing  to 
the  repulsion  between  this  induced  current  and  the 
primary  current,  there  is  a  stoppage  or  disappearance  of 
the  energy  of  actual  motion  of  the  moving  coil.  We 
have,  in  fact,  the  creation  of  one  species  of  energy,  and  at 
the  same  time  the  disappearance  of  another,  and  thus  we 
see  that  the  law  of  conservation  is  by  no  means  broken. 
141.  We  see  also  the  necessary  connection  between  the 


TKANSMUTATIONS   OF   ENERGY.  101 

two  electrical  laws  described  in  Arts.  100  and  lOi.  In- 
deed, had  these  laws  been  other  than  what  they  are,  the 
principle  of  conservation  of  energy  would  have  been 
broken. 

For  instance,  had  the  induced  current  in  the  case  now 
mentioned  been  in  the  same  direction  as  that  of  the 
primary,  the  two  currents  would  have  attracted  each 
other,  and  thus  there  would  have  been  the  creation  of  a 
secondary  current,  implying  energy,  in  the  coil  attached 
to  the  galvanometer,  along  with  an  increase  of  the  visible 
energy  of  motion  of  the  primary  current — that  is  to  say, 
instead  of  the  creation  of  one  kind  of  energy,  accom- 
panied with  the  disappearance  of  another,  we  should 
have  had  the  simultaneous  creation  of  both ;  and  thus 
the  law  of  conservation  of  energy  would  have  been 
broken. 

We  thus  see  that  the  principle  of  conservation  enables 
us  to  deduce  the  one  electrical  law  from  the  other,  and 
this  is  one  of  the  many  instances  which  strengthen  our 
belief  in  the  truth  of  the  groat  principle  for  which  we 
are  contending. 

142.  Let  us  next  consider  what  wiU  take  place  if  we 
cause  the  primary  current  to  move  from  the  secondary 
coil  instead  of  towards  it. 

In  this  case  we  know,  from  Ai-t.  104^,  that  the  induced 
current  will  be  in  the  same  direction  as  the  primary, 
while  we  are  told  by  Ai't.  100  that  the  two  currents  will 
now  attract  each  other.     The  tendency  of  this  attraction 


102  THE   CONSERVATION   OF   ENERGY. 

u'ill  be  to  stop  the  motion  of  tlie  primary  current  from 
the  secondary  one,  or,  in  other  words,  there  will  be  a  dis- 
appearance of  the  energy  of  visible  motion,  while  at  the 
same  time  there  is  the  production  of  a  current.  In  both 
cases,  therefore,  one  form  of  energy  disappears  while 
another  takes  its  place,  and  in  both  there  will  be  a  very 
l^erceptible  resistance  experienced  in  moving  the 
jirimary  coil,  whether  towards  the  secondary  or  from  it. 
Work  will,  in  fact,  have  to  be  spent  in  both  operations, 
and  the  outcome  of  this  work  or  energy  will  be  the  pro- 
duction of  a  current  in  the  first  place,  and  of  heat  in  the 
second;  for  we  learn  from  Art.  98  that  when  a  current 
passes  along  a  wire  its  energy  is  generally  spent  in  heating 
the  wire. 

We  have  thus  tAvo  phenomena  occurring  together.  lu 
the  first  place,  in  moving  a  current  of  electricity  to  and 
from  a  coil  of  wire,  or  any  other  conductor,  or  (which  is 
the  same  thing,  since  action  and  reaction  are  equal  and 
opposite)  in  moving  a  coil  of  wire  or  any  other  con- 
ductor to  and  from  a  current  of  electricity,  a  sense 
of  resistance  will  be  experienced,  and  energy  will  have 
to  be  spent  upon  the  process ;  in  the  second  place,  an 
electrical  current  will  be  generated  in  the  conductor,  and 
the  conductor  will  be  heated  in  consequence. 

143.  The  result  will  be  rendered  very  prominent  if 
we  cause  a  metallic  top,  in  rapid  rotation,  to  spin  near 
two  iron  poles,  which,  by  means  of  the  battery,  we  can 
suddenly  convert  into  the  poles  of  a  powerful  electro- 


TRANSMUTATIONS   OF  ENERGY.  103 

magnet.  When  this  change  is  made,  and  the  poles  be- 
come magnetic,  the  motion  of  the  top  is  very  speedily- 
brought  to  rest,  just  as  if  it  had  to  encounter  a  sj)ecies 
of  invisible  friction.  This  curious  result  can  easily  be 
explained.  We  have  seen  from  Art.  101  that  a  magnet 
resembles  an  assemblage  of  electric  currents,  and  in  the 
metallic  top  we  have  a  conductor  alternately  approaching 
these  currents  and  receding  from  them  ;  and  hence,  ac- 
cording to  what  has  been  said,  we  shall  have  a  series  of 
secondary  currents  produced  in  the  conducting  top  which 
will  stop  its  motion,  and  which  will  ultimately  take  the 
shape  of  heat.  In  other  words,  the  visible  energy  of  the 
top  will  be  changed  into  heat  just  as  truly  as  if  it  were 
stopped  by  ordinary  friction. 

144.  The  electricity  induced  in  a  metallic  conductor, 
moved  in  the  presence  of  a  powerful  magnet,  has  received 
the  name  of  Magneto-Electricity;  and  Dr.  Joule  has 
made  use  of  it  as  a  convenient  means  of  enablincr  him 
to  determine  the  mechanical  equivalent  of  heat,  for  it 
is  into  heat  that  the  energy  of  motion  of  the  conductor 
is  ultimately  transformed.  But,  besides  all  this,  these 
currents  form,  perhaps,  the  very  best  means  of  obtaining 
electricity  ;  and  recently  very  powerful  machines  have 
been  constructed  by  Wild  and  others  with  this  view. 

145.  These  machines,  when  large,  are  worked  by  a 
steam-engine,  and  their  mode  of  operation  is  as  follows: — 
The  nucleus  of  the  machine  is  a  sj'stem  of  powerful 
permanent  steel  magnets,  and  a  conducting  coil  is  made 


104  THE   CONSERVATION    OF   ENERGY. 

to  revolve  rapidly  in  presence  of  tliese  magnets.  The 
current  produced  by  this  moving  coil  is  then  used  in 
order  to  produce  an  extremely  powerful  electro-magnet, 
and  finally  a  coil  is  made  to  move  with  great  rapidity 
in  presence  of  this  powerful  electro-magnet,  thus  causing 
induced  currents  of  vast  strength.  So  pow^erful  are  these 
currents,  that  "when  used  to  produce  the  electric  light, 
small  print  may  be  read  on  a  dark  night  at  the  distance 
of  two  miles  from  the  scene  of  operation  ! 

It  thus  appears  that  in  this  machine  a  doiible  use  is 
made  of  magneto-electricity.  Starting  with  a  nucleus 
of  permanent  magnetism,  the  magneto-electric  currents 
are  used,  in  the  first  instance,  to  form  a  powerful  electro- 
magnet much  stronger  than  the  first,  and  this  powerfid 
electro-magnet  is  again  made  use  of  in  the  same  way  as 
the  first,  in  order  to  give,  by  means  of  magneto- 
electricity,  an  induced  current  of  very  great  strength. 

14G.  There  is,  moreover,  a  very  gi-eat  likeness  between 
a  mascneto-electric  machine  like  that  of  Wild's  for  srene- 
rating  electric  currents,  and  the  one  which  generates 
statical  electricity  by  means  of  the  method  already  de- 
scribed Art.  139.  In  both  cases  advantage  is  taken  of  a 
nucleus,  for  in  the  magneto-electric  machine  we  have 
the  molecular  currents  of  a  set  of  permanent  magnets, 
which  are  made  the  means  of  generating  enormous 
electric  currents  without  any  permanent  alteration  to 
themselves,  yet  not  without  the  expenditure  of  work. 

Again,  in  an  induction  machine  for  generating  statical 


TRANSMUTATIONS  OF   ENEllGV.  105 

electricity,  \vc  liave  an  electric  nucleus,  sucli  as  we  have 
supposed  to  reside  in  the  prime  conductor  of  a  machine  ; 
and  advantage  may  bo  taken,  as  we  have  seen,  of  this 
nucleus  in  order  to  generate  a  vast  quantity  of  statical 
electricity,  without  any  permanent  alteration  of  the 
nucleus,  but  not  without  the  expenditui-e  of  work. 

l-tT.  We  have  now  seen  under  wliat  conditions  the 
visible  energy  of  actual  motion  may  be  changed — Istly, 
into  energy  of  position;  2ndly,  into  the  two  energies 
which  embrace  absorbed  heat ;  Srdly,  into  electrical  sepa- 
ration ;  and  finally  into  electricity  in  motion.  As  far  as 
we  know,  visible  energy  cannot  directly  be  transformed 
into  chemical  separation,  or  into  radiant  energy. 

Visihle  Energy  of  Position. 

148.  Having  thus  exhausted  the  transmutations  of  the 
energy  of  visible  motion,  we  next  turn  to  that  of 
position,  and  find  that  it  is  transmuted  into  motion,  but 
not  immediately  into  any  other  form  of  energy;  we  may, 
therefore,  dismiss  this  variety  at  once  from  our  considera- 
tion. 

Ahsorhed  Heat 

l^d.  Coming  noAV  to  these  two  forms  of  energy  which 
embiuce  ahsorhed  heat,  we  find  that  this  may  be  con- 
verted into  (A)  or  actual  visihle  energy  in  the  case  of 
the  steam-engine,  the  air-engine,  and  all  varieties  of  heat 
engines.     In  the  steam-engine,  for  instance,  part  of  the 


106  THE   CONSERVATION  OF  ENERGY. 

heat  whicli  passes  through  it  disappears  as  heat,  utterly 
and  absolutely,  to  reappear  as  mechanical  effect.  There 
is,  however,  one  condition  which  must  be  rigidly  ful- 
filled, whenever  heat  is  changed  into  mechanical  effect — 
there  must  be  a  difference  of  temperature,  and  heat  will 
only  he  changed  into  U'orJc,  while  it  passes  from  a  hody 
of  high  temperature  to  one  of  low. 

Carnot,  the  celebrated  Frencb  physicist,  has  ingeniously 
likened  the  mechanical  power  of  heat  to  that  of  water ; 
for  just  as  you  can  get  no  work  out  of  heat  unless  there 
be  a  flow  of  heat  from  a  higher  temperature  level  to  a 
lower,  so  neither  can  you  get  work  out  of  water  unless  it 
be  falling  from  a  higher  level  to  a  lower. 

150.  If  we  reflect  that  heat  is  essentially  distributive 
in  its  nature,  we  shall  soon  perceive  the  reason  for  this 
peculiar  law ;  for,  in  virtue  of  its  nature,  heat  is  always 
rushing  from  a  body  of  high  temperature  to  one  of  low, 
and  if  left  to  itself  it  would  distribute  itself  equally 
amongst  all  bodies,  so  that  they  would  ultimately  bc' 
come  of  the  same  temperature.  Now,  if  we  are  to  coax 
work  out  of  heat,  we  must  humour  its  nature,  for  it  may 
be  compared  to  a  •  pack  of  schoolboys,  who  are  always 
ready  to  run  with  sufficient  violence  out  of  the  school- 
room into  the  open  fields,  but  who  have  frequently  to  be 
dragged  back  with  a  very  considerable  expenditure  of 
energy.  So  heat  will  not  allow  itself  to  be  confined, 
but  will  resist  any  attempt  to  accumulate  it  into  a 
limited  space.     Work   cannot,   therefore,  be   gained  by 


TRANSMUTATIONS  OF  ENERGY.  1()7 

Buch  an  operation,  but  must,  on  the  contraiy,  be  Kpe)it 
upon  the  process. 

151.  Let  us  now  for  a  moment  consider  the  case  of  an 
enclosure  in  which  everything  is  of  the  same  temperature. 
Here  we  have  a  dull  dead  level  of  heat,  out  of  which  it 
will  be  impossible  to  obtain  the  faintest  semblance  of 
work.  The  temperature  may  even  be  high,  and  there 
may  be  immense  stores  of  heat  energy  in  the  enclosure, 
but  not  a  trace  of  this  is  available  in  the  shape  of  work. 
Taking  up  Carnot's  comparison,  the  water  has  already 
fallen  to  the  same  level,  and  lies  there  without  any 
power  of  doing  useful  work — dead,  in  a  sense,  as  far  as 
visible  energy  is  concerned. 

152.  We  thus  perceive  that,  firstly,  we  can  get  work 
out  of  heat  when  it  passes  from  a  higher  to  a  lower 
temperature,  but  that,  secondly,  we  must  spend  work  upon 
it  in  order  to  make  it  pass  from  a  lower  temperature  to  a 
higher  one  ;  and  that,  thirdly  and  finally,  nothing  in  the 
shape  of  work  can  be  got  out  of  heat  which  is  all  at  the 
same  temperature  level. 

What  we  have  now  said  enables  us  to  realize  the  con- 
ditions under  which  all  heat  engines  work.  The  essential 
point  about  such  engines  is,  not  the  possession  of  a 
cylinder,  or  piston,  or  fly  wheels,  or  valves,  but  the 
possession  of  two  chambers,  one  of  high  and  the  other 
of  low  temperature,  while  it  performs  work  in  the  process 
of  carrying  heat  from  the  chamber  of  high  to  that  of  \o^Y 
temperature. 


108  THE   CONSERVATION    OF   ENERGY. 

Let  VIS  take,  for  example,  the  low-pressure  engine. 
Here  we  have  tlie  boiler  or  cliamber  of  high,  and  the 
condenser  or  chamber  of  low,  temperature,  and  the  engine 
works  while  heat  is  being  carried  from  the  boiler  to  the 
condenser — never  while  it  is  being  carried  from  the  con- 
denser to  the  boiler. 

In  like  manner  in  the  locomotive  we  have  the  steam 
generated  at  a  high  temperature  and  pressure,  and  cooled 
by  injection  into  the  atmosphere. 

153.  But,  leaving  formal  engines,  let  us  take  an 
ordinary  fii'e,  which  plays  in  truth  the  part  of  an  engine, 
as  far  as  energy  is  concerned.  We  have  here  the  cold 
air  streaming  in  over  the  floor  of  the  room,  and  rushing 
into  the  fire,  to  be  there  united  with  carbon,  while  the 
rarefied  product  is  carried  up  the  chimney.  Dismissing 
from  our  thoughts  at  present  the  process  of  combustion, 
except  as  a  means  of  supplying  heat,  we  see  that  there 
is  a  continual  in-draught  of  cold  air,  which  is  heated  by 
the  tire,  and  then  sent  to  mingle  with  the  air  above. 
Heat  is,  in  fact,  distributed  by  this  means,  or  carried  from 
a  body  of  high  temperature,  i.e.  the  fire,  to  a  body' of  low 
temperature,  i.e.  the  outer  air,  and  in  this  process  of  dis- 
tribution mechanical  effect  is  obtained  in  the  up-rush 
of  air  through  the  chimney  with  considerable  velocity. 

154.  Our  OAvn  earth  is  another  instance  of  such  an 
engine,  having  the  equatorial  regions  as  its  boiler, 
and  the  polar  regions  as  its  condensers  ;  for,  at 
the    equator,    the    air    is    heated    by    the    direct    ra^^^ 


TRANSMUTATIONS  OF  ENERGY.  109 

of  the  sun,  and  we  have  there  an  ascendin^;^  current  of 
air,  up  a  chimney  as  it  were,  the  place  of  which  is  sup- 
plied by  an  in-draught  of  colder  air  along  the  ground 
or  floor  of  the  world,  from  the  poles  on  both  sides.  Thus 
the  heated  air  makes  its  way  from  the  equt^tor  to  the 
poles  in  the  upper  regions  of  the  atmosphere,  while  the 
cold  air  makes  its  way  from  the  poles  to  the  equator 
along  the  lower  regions.  Very  often,  too,  aqueous  vapour 
as  well  as  air  is  carried  up  by  means  of  the  sun's  heat 
to  the  upper  and  colder  atmospheric  regions,  and  there 
deposited  in  the  shape  of  rain,  or  hail,  or  snow,  which 
ultimately,  finds  its  way  back  again  to  the  earth,  often 
displaying  in  its  passage  immense  mechanical  energy. 
Indeed,  the  mariner  who  hoists  his  sail,  and  the  miller 
who  grinds  his  corn  (whether  he  use  the  force  of  the 
\7ind  or  that  of  running  water),  are  both  dependent 
upon  this  great  earth-engine,  which  is  constantly  at  work 
producing  mechanical  effect,  but  always  in  the  act  of 
carrying  heat  from  its  hotter  to  its  colder  regions. 

155.  Now,  if  it  be  essential  to  an  engine  to  have  two 
chambers,  one  hot  and  one  cold,  it  is  equally  important 
that  there  should  be  a  considerable  temperature  differ- 
ence between  the  two. 

K  Nature  insists  upon  a  difference  before  she  will  givo 
us  work,  we  shall  not  be  able  to  pacify  her,  or  to  meet 
her  requirements  by  making  this  difference  as  small  as 
possible.  And  hence,  cceteris  "paribus,  we  shall  obtain  a 
greater  proportion  of  work  out  of  a  certain  amount  of 


110  THE   COXSERVATION   OF  ENERGY. 

heat  passing  tlirougli  our  engine  when  the  temperature 
difference  between  its  boiler  and  condenser  is  as  great 
as  possible.  In  a  steam-engine  this  difference  cannot 
be  very  great,  because  if  the  water  of  the  boiler  were  at 
a  very  high  temperature  the  pressure  of  its  steam  would 
become  dangerous ;  but  in  an  air-engine,  or  engine  that 
heats  and  cools  air,  the  temperature  difference  may  be 
much  larger.  There  are,  however,  practical  inconveniences 
in  engines  for  which  the  temperature  of  the  boiler  is 
very  high,  and  it  is  possible  that  these  may  prove  so 
formidable  as  to  turn  the  scale  against  such  engines, 
although  in  theory  they  ought  to  be  very  economical. 

156.  The  principles  now  stated  have  been  employed  by 
Professor  J.  Thomson,  in  his  suggestion  that  the  appli- 
cation of  pressure  would  be  found  to  lower  the  freezing 
point  of  water ;  and  the  truth  of  this  suggestion  was  after- 
wards proved  by  Professor  Sir  W.  Thomson.  The  fol- 
lowing was  the  reasoning  employed  by  the  former : — 

Suppose  that  we  have  a  chamber  kept  constantly  at 
the  temperature  0°  C,  or  the  melting  point  of  ice,  and 
that  we  have  a  cylinder,  of  which  the  sectional  area 
is  one  square  metre,  filled  one  metre  in  height  with 
water,  that  is  to  say,  containing  one  cubic  metre  of 
water.  Suppose,  next,  that  a  well-fitting  piston  is 
placed  above  the  surface  of  the  water  in  this  cylinder, 
and  that  a  considerable  weight  is  placed  upon  the  piston 
Let  us  now  take  the  cylinder,  water  and  all,  and  carry 
it  into  another  room,  of  which  the  temperature  is  just 


TRANSMUTATIOXS   OF   ENERGY.  Ill 

a  trifle  lo^yer,  In  course  of  time  the  water  will  freeze, 
and,  as  it  expands  in  freezing,  it  will  push  up  the  piston  and 
weight  about  ^fo^hs  of  a  metre;  and  we  may  suppose 
that  the  piston  is  kept  fastened  in  this  position  by  means 
of  a  peg.  Now  carry  back  the  machine  into  the  first 
room,  and  in  the  course  of  time  the  ice  will  be  melted, 
and  we  shall  have  water  once  more  in  the  cylinder,  but 
there  will  now  be  a  void  space  of  i^o^hs  of  a  metre 
between  the  piston  and  the  surface.  We  have  thus  ac- 
quired a  certain  amount  of  energy  of  position,  and  we 
have  only  to  pull  out  the  peg,  and  allow  the  piston  with 
its  weight  to  fall  do^^ai  through  the  vacant  space,  in  order 
to  utilize  this  energy,  after  which  the  arrangement  is  ready 
to  start  afresh.  Again,  if  the  weight  be  very  great,  the 
energy  thus  gained  will  be  very  great ;  in  fact,  the  energy 
will  vary  with  the  weight.  In  fine,  the  arrangement 
now  described  is  a  veritable  heat  engine,  of  which  the 
chamber  at  0°  C.  corresponds  to  the  boiler,  and  the  other 
chamber  a  trifle  lower  in  temperature  to  the  condenser, 
while  the  amount  of  work  we  get  out  of  the  engine — or,  in 
other  words,  its  efficiency — will  depend  upon  the  weight 
which  is  raised  through  the  space  of  i^jths  of  a  metre, 
so  that,  by  increasing  this  weight  without  limit,  we  may 
increase  the  efficiency  of  our  engine  without  limit.  It 
would  thus  at  first  sight  appear  that  by  this  device  of  hav- 
ing two  chambers,  one  at  0°  C,  and  the  other  a  trifle  lower, 
we  can  get  any  amount  of  work  out  of  our  water  engine  ; 
and  that,  consequently,  we  have  managed  to   overcome 

6 


112  THE   CONSERVATION   OF  ENERGI-, 

Nature.  But  here  Thomson's  law  comes  iuto  operation, 
showing  that  we  cannot  overcome  Nature  by  any  such 
device,  but  that  if  we  have  a  large  weight  upon  our 
piston,  we  must  have  a  proportionally  large  diflerence  of 
temperature  between  our  two  chambers — that  is  to  say, 
the  freezing  point  of  water,  under  great  pressure,  will  be 
lower  in  temperature  than  its  freezing  point,  if  thf 
pressure  upon  it  be  only  small. 

Before  leaving  this  subject  we  must  call  upon  our 
readers  to  realize  what  takes  place  in  all  heat  engines. 
It  is  not  merely  that  heat  produces  mechanical  effect, 
but  that  a  given  quantity  of  heat  absolutely  passes  out 
of  existence  as  heat  in  producing  its  equivalent  of  work 
If,  therefore,  we  could  measure  the  mere  heat  produced 
in  an  engine  by  the  burning  of  a  ton  of  coals,  we 
should  find  it  to  be  less  when  the  engine  was  doing 
work  than  when  it  was  at  rest. 

In  like  manner,  when  a  gas  expands  suddenly  its 
temperature  falls,  because  a  certain  amount  of  its  heat 
passes  out  of  existence  in  the  act  of  producing  mechani- 
cal effect. 

157.  We  have  thus  endeavoured  to  show  under  what 
conditions  absorbed  heat  may  be  converted  into  mechani- 
cal effect.  This  absorbed  heat  embraces  (Art.  110)  two 
varieties  of  energy,  one  of  these  being  molecular  motion, 
and  the  other  molecular  energy  of  position. 

Let  us  now,  therefore,  endeavour  to  ascertain  under 
what  circumstances  the  one  of  these  vaiieties  may  be 


TBANSMUTATIONS   OF   ENEKGY.  113 

clianged  into  the  other.  It  is  well  known  that  it  takes 
a  good  deal  of  heat  to  convert  a  kilogramme  of  ice  into 
water,  and  that  when  the  ice  is  melted  the  temperature 
of  the  water  is  not  perceptibly  higher  than  that  of  the 
ice.  It  is  equally  weU  known  that  it  takes  a  great  deal 
of  heat  to  convert  a  kilogramme  of  boiling  water  into 
steam,  and  that  when  the  transformation  is  accomplished, 
the  steam  produced  is  not  perceptibly  hotter  than  the 
boilinor  water.  In  such  cases  the  heat  is  said  to  become 
latent. 

Now,  in  both  these  cases,  but  more  obviously  in  the 
last,  we  may  suppose  that  the  heat  has  not  had  its  usual 
office  to  perform,  but  that,  instead  of  increasing  the 
motion  of  the  molecules  of  water,  it  has  spent  its  energy 
in  tearinof  them  asunder  from  each  other,  ag-ainst  the 
force  of  cohesion  which  binds  them  together. 

Indeed,  we  know  as  a  matter  of  fact  that  the  force  of 
cohesion  which  is  perceptible  in  boiling  water  is  ap- 
parently absent  from  steam,  or  the  vapour  of  water,  because 
its  molecules  are  too  remote  from  one  another  to  allow  of 
this  force  being  appreciable.  We  may,  therefore,  suppose 
that  a  large  part,  at  least,  of  the  heat  necessary  to  con 
vert  boiling  water  into  steam  is  spent  in  doing  work 
against  molecular  forces. 

Wlien  the  steam  is  once  more  condensed  into  hot  water 
the  heat  thus  spent  reassumes  the  form  of  molecular 
motion,  and  the  consequence  is  that  we  require  to  take 
away  somehow  all  the  latent  heat  of  a  kilogTamme  of 


lit  THE   CONSEllVATION   OF  ENERGY. 

6team  before  we  can  convert  it  into  boiling  water.  In 
fact,  if  it  is  difficult  and  tedious  to  convert  water  into 
steam,  it  is  difficult  and  tedious  to  convert  steam  into 
water. 

158.  Besides  the  case  now  mentioned,  there  are  other 
instances  in  which,  no  doubt,  molecular  separation 
becomes  gradually  changed  into  heat  motion.  Thus, 
when  a  piece  of  glass  has  been  suddenly  cooled,  its  par- 
ticles have  not  had  time  to  acquire  their  proper  position, 
and  the  consequence  is  that  the  whole  structure  is  thrown 
into  a  state  of  constraint.  In  the  course  of  time  such 
bodies  tend  to  assume  a  more  stable  state,  and  their 
particles  gradually  come  closer  together. 

It  is  owing  to  this  cause  that  the  bulb  of  a  thermo- 
meter recently  blown  gradually  contracts,  and  it  is  no 
doubt  owing  to  the  same  cause  that  a  Prince  Rupert's 
drop,  formed  by  dropping  melted  glass  into  water,  when 
broken,  falls  into  powder  with  a  kind  of  explosion.  It 
seems  probable  that  an  all  such  cases  these  changes  are 
attended  with  heat,  and  that  they  denote  the  conversion 
of  the  energy  of  molecular  separation  into  that  of 
molecular  motion. 

159.  Having  thus  examined  the  transmutations  of  (C) 
into  (D),  and  of  (D)  back  again  into  (C),  let  us  now 
proceed  with  our  list,  and  see  under  what  circumstances 
absorbed  heat  is  changed  into  cJiemical  seiiaration. 

It  is  well  known  that  when  certain  bodies  are  heated, 
they  are  decomposed ;  for  instance,  if  limestone   or   car- 


TRANSMUTATIONS   OF  ENERGY.  llo 

bonate  of  lime  be  heated,  it  is  decomposed,  the  carhoiiic 
acid  being  given  out  in  the  shape  of  gas,  while  quick- 
lime remains  behind.  Now,  heat  is  consumed  in  this 
process,  that  is  to  say,  a  certain  amount  of  heat  energy 
absolutely  passes  out  of  existence  as  heat  and  is  changed 
into  the  energy  of  chemical  separation.  Again,  if  the 
lime  so  obtained  be  exposed,  under  certain  circum- 
stances, to  an  atmosphere  of  carbonic  acid,  it  will 
gradually  become  changed  into  carbonate  of  lime ;  and  in 
this  change  (which  is  a  gradual  one)  we  may  feel  assured 
that  the  energy  of  chemical  separation  is  once  more  con- 
verted into  the  energj^  of  heat,  although  we  may  not  per- 
ceive any  increment  of  temperature,  on  account  of  the 
blow  nature  of  the  process. 

At  very  high  temperatures  it  is  possible  that  most 
compounds  are  decomposed,  and  the  temperature  at 
which  this  takes  place,  for  any  compound,  has  been 
termed  its  temijeratiire  of  disassociation. 

IGO.  Heat  energy  is  changed  into  electrical  sejKtration 
when  tourmalines  and  certain  other  crystals  are  heated. 

Let  us  take,  for  instance,  a  crystal  of  tourmaline  and 
raise  its  temperature,  and  we  shall  find  one  end  positively, 
and  the  other  negatively,  electrified.  Again,  let  us  take 
the  same  crystal,  and  suddenly  cool  it,  and  we  shall  find 
an  electrification  of  the  opposite  kind  to  the  former,  so 
that  the  end  of  the  axis,  which  was  then  positive,  will 
now  be  negative.  Now,  this  separation  of  the  electricities 
denotes  energy  ;   and  we  have,  therefore,  in  such  crystals 


116 


THE   CONSERVATION   OF   ENERGY. 


c  •< 


a  case  wliere  the  energy  of  heat  has  been  changed  into 
that  of  electrical  separation.  In  other  words,  a  certain 
amount  of  heat  has  passed  out  of  existence  as  heat, 
v/hile  in  its  place  a  certain  amount  of  electrical  separa- 
tion has  been  obtained, 

161.  Let  us  next  see  under  what  circumstances  heat  is 
changed  into  electricity  in  motion.  This  transmutation 
takes  place  in  thermo-electricity. 

Suppose,  for  instance,  that  we  have  a  bar  of  copper  or 
antimony,  say  copper,  soldered 
to  a  bar  of  bismuth,  as  in  Fig. 
12.  Let  us  now  heat  one  of 
the  junctions,  while  the  other 
remains  cool.  It  will  be  found 
that  a  current  of  positive  elec- 

.,.„«,.i„...M,.:.«,... .,.^M^   tricity    circulates    round   the 

Fig.  12.  bar,  in  the  direction   of  the 

arrow-head,  going  from  the  bismuth  to  the  copper  across 
the  heated  junction,  the  existence  of  which  may  be 
detected  by  means  of  a  compass  needle,  as  we  see  in  the 
figure. 

Here,  then,  we  have  a  case  in  which  heat  energy 
goes  out  of  existence,  and  is  converted  into  that  of  an 
electric  current,  and  we  may  even  arrange  matters 
so  as  to  make,  on  this  principle,  an  instrument  which 
shall  be  an  extremely  delicate  test  of  the  existence  of 
heat. 

By  having  a  number    of  junctions   of  bismuth  and 


TRANSMUTATIONS  OF   ENERGY. 


117 


nntiinony,  as  in  Fig.  13,  and  heating  the  upper  set,  while 
the  lower  remain  cool,  we  get  a 
strong  cui'rent  going  from  the  bis- 
muth to  the  antimony  across  the 
heated  junctions,  and  we  may  pass 
the  current  so  pi'oduced  round  the 
wire  of  a  galvanometer,  and  thus, 
by  increasing  the  number  of  our 
j  unctions,  and  also  by  using  a  very  ^v 


Fig.  13. 

This  arrange- 


delicate  galvanometer,  we  may  get 

a  very  perceptible   effect  for    the 

smallest  heating  of  the  upper  junctions. 

ment  is  called  the  thevTinopile,  and,  in  conjiuiction  with 

the  reflecting  galvanometer,  it  affords  the  most  delicate 

means  known  for  detecting  small  quantities  of  heat. 

1G2.  The  last  transmutation  on  our  list  with  respect  to 
absorbed  heat  is  that  in  which  this  species  of  energy  is 
transformed  into  radiant  light  and  heat.  This  takes 
place  whenever  a  hot  body  cools  in  an  open  space — the 
sun,  for  instance,  parts  with  a  large  quantity  of  his  heat 
in  this  way ;  and  it  is  due,  in  part  at  least,  to  this  process 
that  a  hot  body  cools  in  air,  and  wholly  to  it  that  such  a 
body  cools  in  vacuo.  It  is,  moreover,  due  to  the  pene- 
tration of  our  eye  by  radiant  energy  that  we  are  able  to 
Bee  hot  bodies,  and  thus  the  very  fact  that  we  see  them 
implies  that  they  are  parting  with  their  heat. 

Kadiant  energy  moves  through  space  with  the  enormous 
velocity  of  188,000  miles  in  one  second.     It  takes  about 


118  THE   CONSERVATION   OF   ENERGY. 

eight  minutes  to  come  from  the  sun  to  our  earthy  so  thai 
if  our  himinary  were  to  be  suddenly  extinguished,  we 
should  have  eight  minutes,  respite  before  the  catastrophe 
overtook  us.  Besides  the  rays  that  affect  the  eye,  there 
are  others  which  we  cannot  see,  and  which  may  therefore 
be  termed  dark  rays.  A  body,  for  instance,  may  not  be 
hot  enough  to  be  self-luminous,  and  yet  it  may  be  rapidly 
cooling  and  changing  its  heat  into  radiant  energy,  which 
is  given  oft'  by  the  body,  even  although  neither  the  eye 
nor  the  touch  may  be  competent  to  detect  it.  It  may 
nevertheless  be  detected  by  the  thermopile,  Avhich  was 
described  in  Art.  IGl.  We  thus  isee  how  strong  is  the 
likeness  between  a  heated  body  and  a  sounding  one. 
For  just  as  a  sounding  body  gives  out  part  of  its  sound 
energy  to  the  atmosphere  around  it,  so  does  a  heated 
body  give  out  part  of  its  heat  energy  to  the  ethereal 
medium  around  it.  When,  however,  we  consider  the 
rates  of  motion  of  these  energies  throucrh  their  re- 
spective  media,  there  is  a  mighty  difference  between 
the  two,  sound  travelling  through  the  air  with  the 
velocity  of  1100  feet  a  second,  while  radiant  energy 
moves  over  no  less  a  space  than  1 88,000  miles  in  the 
same  portion  of  time. 

Chemical  Scj)araiion. 

1C3.  We  now  come  to  the  energy  denoted  by  chemical 
separation,  such  as  we  possess  when  we  have  coal  or 
caibon  in  one  place,  and  oxygen  in  another.     Very  cvi- 


TRANSMUTATIONS   OF   ENERGY.  119 

dently  this  form  of  energy  of  position  is  transmuted  into 
heat  when  we  burn  the  coal,  or  cause  it  to  combine  with 
the  oxygen  of  the  air ;  and  generally,  whenever  chemical 
combination  takes  place,  we  have  the  production  of  heat 
even  although  other  circumstances  may  interfere  to  pre- 
vent its  recognition. 

Now,  in  accordance  with  the  principle  of  conservation, 
it  may  be  expected  that,  if  a  definite  quantity  of  carbon 
or  of  hydrogen  be  burned  under  given  circumstances, 
there  will  be  a  definite  production  of  heat;  that  is  to 
say,  a  ton  of  coals  or  of  coke,  when  burned,  will  give  us 
so  many  heat  units,  and  neither  more  or  less.  We  may, 
no  doubt,  burn  our  ton  in  such  a  way  as  to  economize 
more  or  less  of  the  heat  produced ;  but,  as  far  as  the  mere 
production  of  heat  is  concerned,  if  the  quantity  and 
quality  of  the  material  burned  and  the  circumstances  of 
combustion  be  the  same,  we  expect  the  same  amount  of 
heat. 

IGl.  The  following  table,  derived  from  the  researches 

of  Andrews,  and  those  of  Favre  and  Silbermann,  shows 

us  how  many  units  of  heat  we  may  get  by  burning  a 

kilogramme  of  various  substances. 

Units  of  Heat  developed  h]/  Combustion  in  Oxygen, 

Kilogrammes  of  Water  raised  1°  C. 
Sub.^taTice  by  the  combustion  of  one  kilo- 

Earned.  gramme  of  each  substance. 

Hydrogen     S^'jlSo 

Carbon 7,990 

Sulphur    2.2G3 


120  THE   CONSERVATION   OF   ENERGY. 

Kilogrammes  of  Water  raised  1°  C. 
Substance  by  the  combustion  of  one  kilo> 

Bui-ned,  gramme  of  each  substance. 

PhosphoriTs 5,747 

Zinc 1,301 

Iron 1,576 

Tin   1,233 

OlefiantGas     11,900 

Alcohol 7,016 

165.  There  are  other  methods,  besides  combustion,  by 
which  chemical  combination  takes  place. 

When,  for  instance,  we  plunge  a  piece  of  metallic  iron 
into  a  solution  of  copper,  we  find  that  when  we  take  it 
out,  its  surface  is  covered  with  copper.  Part  of  the  iron 
has  been  dissolved,  taking  the  place  of  the  copper,  which 
has  therefore  been  thrown,  in  its  metallic  state,  upon  the 
surface  of  the  iron.  Now,  in  tlws  operation  heat  is  given 
out — we  have  in  fact  burned,  or  oxidized,  the  iron,  and 
we  are  thus  furnished  with  a  means  of  arranoinof  the 
metals,  beginning  with  that  which  gives  out  most  heat, 
when  used  to  displace  the  metal  at  the  other  extremity 
of  the  series. 

166.  The  following  list  has  been  formed,  on  this  prin- 
ciple, by  Dr.  Andrews  : — 

1.  Zinc  5.  Mercury 

2.  Iron  6.  Silver 

3.  Lead  7.  Platinum 

4.  Copper 


TRANSMUTATIONS   OF   ENERGY.  121 

— that  is  to  say,  tlie  metal  platinum  can  be  displaced  by 
any  other  metal  of  the  series,  but  we  shall  get  most  heat 
if  we  use  zinc  to  displace  it. 

We  may  therefore  assume  that  if  we  displace  a  defi- 
nite quantity  of  platinum  by  a  definite  quantity  of  zinc, 
we  shall  get  a  definite  amount  of  heat.  Suppase, 
however,  that  instead  of  performing  the  operation  in  one 
step,  we  make  two  of  it.  Let  us,  for  instance,  first  of  all 
displace  copper  by  means  of  zinc,  and  then  platinum  by 
means  of  copper.  Is  it  not  possible  that  the  one  of  these 
processes  may  be  more  fruitful  in  heat  giving  than  the 
other  ?  Now,  Andrews  has  shown  us  that  we  cannot 
gain  an  advantage  over  Nature  in  this  way,  and  that  if 
we  use  our  zinc  first  of  all  to  displace  iron,  or  copper,  or 
lead,  and  then  use  this  metal  to  displace  platinum,  we 
shall  obtain  just  the  very  same  amount  of  heat  as  if  we 
had  used  the  zinc  to  displace  the  platinum  at  once. 

167.  It  ought  here  to  be  mentioned  that,  very  generally, 
chemical  action  is  accompanied  with  a  change  of 
molecular  condition. 

A  solid,  for  instance,  may  be  changed  into  a  liquid, 
or  a  gas  into  a  liquid.  Sometimes  the  one  change 
counteracts  the  other,  as  far  as  apparent  heat  is  concerned; 
but  sometimes,  too,  they  co-operate  together  to  increase 
the  result.  Thus,  when  a  gas  is  absorbed  by  water, 
much  heat  is  evolved,  and  we  may  suppose  the  result 
to  be  due  in  part  to  chemical  combination,  and  in  part 
to  the  condensation  of  the  gas  into  a  liquid,  by  which 


122        THE  CONSERVATION  OF  ENERGY. 

means  its  latent  heat  is  rendered  sensible.  On  the 
other  hand,  when  a  liquid  unites  with  a  solid,  or  when 
two  solids  unite  with  one  another,  and  the  product 
is  a  liquid,  we  have  very  often  the  absor])tion  of 
heat,  the  heat  rendered  latent  by  the  dissolution  of 
the  solid  being  more  than  that  generated  by  combina- 
tion. Freezing  mixtures  owe  their  cooling  properties 
to  this  cause;  thus,  if  snow  and  salt  be  mixed  to- 
gether, they  liquefy  each  other,  and  the  result  is  brine 
of  a  temperature  much  lower  than  that  of  either  the 
ingredients. 

1G8.  When  heterogeneous  metals,  such  as  zinc  and 
copper,  are  soldered  together,  we  have  apparently  a 
conversion  of  the  energy  of  chemical  separation  into 
that  of  electrical  separation.  This  was  first  suggested 
by  Volta  as  the  origin  of  the  electrical  separation  which 
we  see  in  the  voltaic  current,  and  recently  its  existence 
has  been  distinctly  proved  by  Sir  W.  Thomsoix 

To  render  manifest  this  conversion  of  energy,  let  us 
solder  a  piece  of  zinc  and  copper  together — if  we  now 
test  the  bar  by  means  of  a  delicate  electrometer  we  shall 
find  that  the  zinc  is  positively,  while  the  copper  is  nega- 
tively, electrified.  We  have  here,  therefore,  an  instance 
of  the  transmutation  of  one  form  of  energy  of  j)osition 
into  another;  so  much  energy  of  chemical  separation 
disappearing  in  order  to  produce  so  much  electrical  sepa- 
ration. This  explains  the  fact  recorded  in  Art,  93, 
where  we  saw  that  if  a  battery  be  insulated  and  its  poles 


TRANSMUTATIONS   OF   ENERGY.  123 

kept  apart,  the  one  will  be  charged  with  positive,  and 
the  other  with  negative,  electricity. 

1G9.  But  further,  when  such  a  voltaic  battery  is  in 
action,  we  have  a  transmutation  of  chemical  separation 
into  electricity  in  motion.  To  see  this,  let  us  consider 
what  takes  place  in  such  a  battery. 

Here  no  doubt  the  sources  of  electrical  excitement  are 
the  points  of  contact  of  the  zinc  and  platinum,  where,  as 
we  see  by  our  last  article,  we  have  electrical  separatv^n 
produced.  But  this  of  itself  would  not  piodace  a 
current,  for  an  electrical  current  implies  very  consider- 
able energy,  and  must  be  fed  by  something.  Now,  in 
the  voltaic  battery  we  have  two  things  which  ac- 
company each  other,  and  which  are  manifestly  con- 
nected together.  In  the  first  place  we  have  the  com- 
bustion, or  at  least  the  oxidation  and  dissolution,  of 
the  zinc ;  and  we  have,  secondly,  the  production  of  a 
powerful  current.  Now,  e^ddently,  the  first  of  these  is 
that  which  feeds  the  second,  or,  in  other  words,  the 
energy  of  chemical  separation  of  the  metallic  zinc  is 
transmuted  into  that  of  an  electrical  current,  the  zinc 
being  virtually  burned  in  the  process  of  transmutatioru 

170.  Finally,  as  far  as  we  are  aware,  the  energy  of 
chemical  separation  is  not  directly  transmuted  into 
radiant  light  and  heat. 


124  TUE   CONSERVATION   OF   ENERGY, 

Electrical  Separation. 

171.  In  tlie  first  place  the  energy  of  electrical  separa- 
tion is  obviously  transmuted  into  that  of  visible  motion, 
when  two  oppositely  electrified  bodies  approach  each 
other. 

172.  Again,  it  is  transmuted  into  a  current  of 
electricity,  and  ultimately  into  heat,  when  a  spark  passes 
between  two  oppositely  electrified  bodies. 

It  ought,  therefore,  to  be  borne  in  mind  that  when  the 
flash  is  seen  there  is  no  longer  electricity,  what  we  see 
being  merely  air,  or  some  other  material,  intensely  heated 
by  the  discharge.  Thus  a  man  might  be  rendered  in- 
sensible by  a  flash  of  lightning  without  his  seeing  the 
flash — for  the  efi'ect  of  the  discharge  upon  the  man,  and 
its  effect  in  heating  the  air,  might  be  phenomena  so 
nearly  simultaneous  that  the  man  might  become  in- 
sensible before  he  could  perceive  the  flash. 

Electricity  in  Motion. 

173.  This  energy  is  transmuted  into  that  of  visible 
motion  when  two  wires  conveying  electrical  currents  in 
the  same  direction  attract  each  other.  When,  for  in- 
stance, two  circular  currents  float  on  water,  both  going 
in  the  direction  of  the  hands  of  a  watch,  we  have  seen 
from  Art.  100  that  they  will  move  towards  each  other. 
Now,  here  there  is,  in  truth,  a  lessening  of  the  intensity 
of  each   current   when   the  motion  is  taking  place,  fur 


TRANSMUTATIONS   OF   ENEllCV.  125 

we  know  (Art.  lOl-)  that  when  a  circuit  is  moved  into 
the  presence  of  another  circuit  conveying  a  current,, 
there  is  produced  by  induction  a  current  in  the  o]3posite 
direction ;  and  hence  we  perceive  that,  when  two  similar 
currents  approach  each  other,  each  is  diminished  by 
means  of  this  inductive  influence — in  fact,  a  certain 
amount  of  current  energy  disappears  from  existence 
in  order  that  an  equivalent  amount  of  the  energy  of 
visible  motion  may  be  produced. 

174  Electricity  in  motion  is  transmuted  into  heat 
during  the  passage  of  a  current  along  a  thin  wire,  or  any 
badly  conducting  substance — the  wire  is  heated  in  con- 
sequence, and  may  even  become  white  hot.  Most 
frequently  the  energy  of  an  electric  current  is  spent  in 
heating  the  wires  and  other  materials  that  form  the 
circuit.  Now,  the  energy  of  such  a  current  is  fed  by  the 
burning  or  oxidation  of  the  metal  (generally  zinc)  which 
is  used  in  the  circuit,  so  that  the  ultimate  effect  of  this 
combustion  is  the  heating  of  the  various  wires  and  other 
materials  through  which  the  current  passes. 

175.  We  may,  in  truth,  burn  or  oxidize  zinc  in  two 
ways — ^we  may  oxidize  it,  as  we  have  just  seen,  in  the 
voltaic  battery,  and  we  shall  find  that  by  the  combustion 
of  a  kilogramme  of  zinc  a  definite  amount  of  heat  is 
])roduced.  Or  we  may  oxidize  our  zinc  by  dissolving  it 
in  acid  in  a  single  vessel,  when,  without  going  through  the 
intermediate  process  of  a  current,  we  shall  get  just  as 
much  heat  out  of  a  kiloo-ramme  of  zinc  as  we  did  in  the 


126  THE   CONSERVATION  OF  ENERGY. 

former  case.  In  fact,  whether  we  oxidize  our  zinc  by  tlio 
battery,  or  in  the  ordinary  way,  the  quantity  of  heat 
produced  will  always  bear  the  same  relation  to  the 
quantity  of  zinc  consumed;  the  only  difference  being 
that,  in  the  ordinary  way  of  oxidizing  zinc,  the  heat  is 
generated  in  the  vessel  containing  the  zinc  and  acid, 
while  in  the  battery  it  may  make  its  appearance  a 
thousand  miles  away,  if  we  have  a  suflficiently  long  wire 
to  convey  our  current. 

176.  This  is,  perhaps,  the  right  place  for  alluding  to  a 
discovery  of  Peltier,  that  a  current  of  positive  electricity 
passing  across  a  junction  of  bismuth  and  antimony  in 
the  direction  from  the  bismuth  to  the  antimony  appears 
to  produce  cold. 

To  understand  the  significance  of  this  fact  we  must 
consider  it  in  connection  with  the  thermo-electric 
current,  which  we  have  seen,  from  Art.  161,  is  established 
in  a  circuit  of  bismuth  and  antimony,  of  which  one 
iunction  is  hotter  than  the  other.  Suppose  we  have  a 
circuit  of  this  kind  with  both  its  junctions 
at  the  temperature  of  100°  C  to  begin  with. 
Suppose,  next,  that  while  we  protect  one 
junction,  we  expose  the  other  to  the  open 
air — it  will,  of  course,  lose  heat,  so  that 
the  protected  junction  will  now  be  hotter 
than  the  other.  The  consequence  will  be 
(Art.  161)  that  a  current  of  positive  elec- 
tricity will  pass  along  the  protected  junc- 
tion from  the  bismuth  to  the  antimony. 


TRANS.MUTATIONS   OF   ENEHGY.  127 

Now,  here  we  have  an  apparent  anomaly,  for  the 
circuit  is  cooling — that  is  to  say,  it  is  losing  energy 
• — but  at  the  very  same  time  it  is  manifesting  energy 
in  another  shape,  namely,  in  that  of  an  electric  current, 
which  is  circulating  round  it.  Clearly,  then,  some  of 
the  heat  of  this  circuit  must  be  spent  in  generating 
this  current ;  in  fact,  we  should  expect  the  circuit  to 
act  as  a  heat  engine,  only  producing  current  energy 
instead  of  mechanical  energy,  and  hence  (Art.  152)  we 
should  expect  to  see  a  conveyance  of  heat  from  the 
hotter  to  the  colder  parts  of  the  circuit.  Now,  this  is 
precisely  what  the  current  does,  for,  passing  along  the 
hotter  junction,  in  the  direction  of  the  arrow-head,  it 
cools  that  junction,  and  heats  the  colder  one  at  c, — in 
other  words,  it  carries  heat  from  the  hotter  to  the  colder 
parts  of  the  circuit.  We  should  have  been  very  much 
surprised  had  such  a  current  cooled  C  and  heated  H, 
for  then  we  should  have  had  a  manifestation  of  current 
energy,  accompanied  with  the  conveyance  of  heat  from  a 
colder  to  a  hotter  substance,  which  is  against  the  principle 
of  Art.  152. 

177.  Finally,  the  energy  of  electricity  in  motion  is 
converted  into  that  of  cJiemical  separation,  when  a 
current  of  i;lectricity  is  made  to  decompose  a  body. 
Part  of  the  energy  of  the  current  is  spent  in  this  process, 
and  we  shall  get  so  much  less  heat  from  it  in  conse- 
quence. Suppose,  for  instance,  that  by  oxidizing  so 
much  zinc  in  the  battery  we  get,  under  ordinary  circum- 


123  THE   CONSERVATION   OF  ENERGY. 

stances,  100  units  of  heat.  Let  us,  however,  set  the 
battery  to  decompose  water,  and  we  shall  probably  find 
that  by  oxidizing  the  same  amount  of  zinc  we  get  now 
only  80  units  of  heat.  Clearly,  then,  the  deficiency  or 
20  units  have  gone  to  decompose  the  water.  Now,  if  we 
explode  the  mixed  gases  which  are  the  result  of  the 
decomposition,  we  shall  get  back  these  20  units  of 
heat  precisely,  and  neither  more  nor  less ;  and  thus  we 
see  that  amid  all  such  changes  the  quantity  of  energy 
remains  the  same. 

Radiant  Entrgy. 

178.  This  form  of  energy  is  converted  into  absorbed 
heat  whenever  it  falls  upon  an  opaque  substance — some  of 
it,  however,  is  generally  conveyed  away  by  reflexion,  but 
the  remainder  is  absorbed  by  the  body,  and  consequently 
heats  it. 

It  is  a  curious  question  to  ask  what  becomes  of  the 
radiant  light  from  the  sun  that  is  not  absorbed  either  by 
the  planets  of  our  system,  or  by  any  of  the  stars.  We 
can  only  reply  to  such  a  question,  that  as  far  as  we  can 
judge  from  our  present  knowledge,  the  radiant  energy 
that  is  not  absorbed  must  be  conceived  to  be  traversing 
space  at  the  rate  of  188,000  miles  a  second. 

179.  There  is  only  one  more  transmutation  of  radiant 
energy  that  we  know  of,  and  that  is  when  it  promotes 
chemical  separation.  Thus,  certain  rays  of  the  sun  are 
known  to  have  the  power  of  decomposing  chloride   of 


TRANSMUTATIONS   OF   ENERGY.  129 

silver,  and  other  elieniical  compounds.  Now,  in  all  sueli 
cases  there  is  a  transmutation  of  radiant  energy  into 
that  of  chemical  separation.  The  sun's  rays,  too,  decom- 
pose carLonic  acid  in  the  leaves  of  plants,  the  carbon 
going-  to  form  the  woody  fibre  of  the  plant,  while  the 
oxygen  is  set  free  into  the  air;  and  of  course  a  certain 
proportion  of  the  energy  of  the  solar  rays  is  consumed 
in  promoting  this  change,  and  we  have  so  much  less 
heating  effect  in  consequence. 

But  all  the  solar  rays  have  not  this  power — for  the 
property  of  promoting  chemical  change  is  confined  to  the 
blue  and  %dolet  ra^'s,  and  some  others  which  are  not 
visible  to  the  eye.  Now,  these  rays  are  entirely  absent 
from  the  radiation  of  bodies  at  a  comparatively  low 
temperature,  such  as  an  ordinary  red  heat,  so  that  a 
photographer  would  find  it  impossible  to  obtain  the 
picture  of  a  red-hot  body,  whose  only  light  was  in  itself 

180.  The  actinic,  or  chemically  active,  rays  of  the  sun 
decompose  carbonic  acid  in  the  leaves  of  plants,  and  they 
disappear  in  consequence,  or  are  absorbed  ;  this  may, 
therefore,  be  the  reason  why  very  few  such  rajs  are  either 
reflected  or  transmitted  from  a  sun-lit  leaf,  in  conse- 
quence of  which  the  photographer  finds  it  difficult  to 
obtain  an  image  of  such  a  leaf;  in  other  words,  the  rays 
which  would  have  produced  a  chemical  change  on  his 
photographic  plate  have  all  been  used  up  by  the  leaf  for 
peculiar  purposes  of  its  own. 

181.  And  here  it  is  important  to  bear  in  mind  that 


130  THE   CONSERVATION   OF  ENERGY. 

while  animals  in  the  act  of  breathing  consume  tli'^ 
oxygen  of  the  air,  turning  it  into  carbonic  acid,  plants, 
on  the  other  hand,  restore  the  oxygen  to  the  air ;  thus 
the  two  kingdoms,  the  animal  and  the  vegetable,  work 
into  each  other's  hands,  and  the  purity  of  tlie  atmosphere 
is  kept  up. 


CHAPTER  V. 

HISTORICAL   SKETCH:     THE  DISSIPATION  OF 
ENERGY. 

182.  In  the  last  chapter  we  have  endeavoured  to  ex- 
hibit the  various  transmutations  of  energy,  and,  while 
doing  so,  to  bring  forward  evidence  in  favour  of  the 
theory  of  conservation,  showing  that  it  enables  us  to 
couple  together  known  laws,  and  also  to  discover  new 
ones — showing,  in  fine,  that  it  bears  about  with  it  all  the 
marks  of  a  true  hypothesis. 

It  may  now,  perhaps,  be  instructive  to  look  back  and 
endeavour  to  trace  the  progress  of  this  great  conception, 
from  its  fii'st  beginning  among  the  ancients,  up  to  its 
triumphant  establishment  by  the  labours  of  Joule  and 
his  fellow- workers. 

183.  Mathematicians  inform  us  that  if  matter  consists 
of  atoms  or  small  parts,  which  are  actuated  by  forces 
depending  only  upon  the  distances  between  these  parts, 
and  not  upon  the  velocity,  then  it  may  be  demonstrated 
that  the  law  of  conservation  of  energy  will  hold  good. 
Thus  we  see  that  conceptions  regarding  atoms  and  their 


132  THE    CONSERVATION    OF   ENERGY. 

forces  are  allied  to  conceptions  regarding  energy.  A 
medium  of  some  sort  pervading  space  seems  also  neces- 
sary to  our  theory.  In  fine,  a  universe  composed  of 
atoms,  with  some  sort  of  medium  between  them,  is  to 
be  regarded  as  the  machine,  and  the  laws  of  energy  as 
the  laws  of  working  of  this  machine.  It  may  be  that 
a  theory  of  atoms  of  this  sort,  with  a  medium  between 
them,  is  not  after  all  the  simplest,  but  we  are  proba- 
bly not  yet  prepared  for  any  more  general  hypothesis. 
Now,  we  have  only  to  look  to  our  o\^ti  solar  system,  in 
order  to  see  on  a  large  scale  an  illustration  of  this  concep- 
tion, for  there  we  have  the  various  heaveidy  bodies  attract-- 
ing  one  another,  with  forces  depending  only  on  the  dis- 
tances between  them,  and  independent  of  the  velocities ; 
and  we  have  likewise  a  medium  of  some  sort,  in  virtue  of 
which  radiant  energy  is  conveyed  from  the  sun  to  the  earth. 
Perhaps  we  shall  not  greatly  err  if  we  regard  a  molecule 
as  representing  on  a  small  scale  something  analogous  to 
the  solar  system,  while  the  various  atoms  which  con- 
stitute the  molecule  may  be  likened  to  the  various  bodies 
of  the  solar  system.  The  short  historical  sketch  which 
we  are  about  to  give  will  embrace,  therefore,  along  with 
energy,  the  progress  of  thought  and  speculation  with 
respect  to  atoms  and  also  with  respect  to  a  medium,  in- 
asmuch as  these  subjects  are  intimately  connected  with 
Lhe  doctrines  of  energy. 


HISTOllICAL   SKETCH.  133 

Heraclitus  on  Energy. 

18  k  Heraclitus,  who  flourished  at  Ephesus,  B.C.  500, 
declared  that  fire  was  the  great  cause,  and  that  all  things 
M^ere  in  a  peipetual  flux.  Such  an  expression  will  no 
doubt  be  regarded  as  very  vague  in  these  days  of  pre- 
cise physical  statements ;  and  yet  it  seems  clear  that 
Heraclitus  must  have  had  a  vivid  conception  of  the 
innate  restlessness  and  energy  of  the  universe,  a  concep- 
tion allied  in  character  to,  and  only  less  pi'ecise  than  that 
of  modern  philosophers,  who  regard  matter  as  essentially 
dynamical. 

Bemocrltus  on  Atoms. 

185.  Democritus,  who  was  born  470  E.C.,  was  the 
originator  of  the  doctrine  of  atoms,  a  doctrine  which  in 
the  hands  of  John  Dalton  has  enabled  the  human  mind 
to  lay  hold  of  the  laws  which  regulate  chemical  changes, 
as  well  as  to  picture  to  itself  what  is  there  taking  place. 
Perhaps  there  is  no  doctrine  that  has  nowadays  a  more 
intimate  connection  with  the  industries  of  life  than  this 
of  atoms,  and  it  is  probable  that  no  intelligent  director  of 
chemical  industry  among  civilized  nations  fails  to  picture 
to  his  own  mind,  by  means  of  this  doctrine,  the  inner 
nature  of  the  changes  which  he  sees  with  his  eyes.  Now, 
it  is  a  curious  circumstance  that  Bacon  should  have 
lighted  upon  this  very  doctrine  of  atoms,  in  order  to 
point  one  of  his  philosophical  morals. 


134!  TUE   CONSERVATION   OF  ENERGY. 

"K^or  is  it  less  an  evil"  (says  lie),  "tliat  in  their  pliilosopkieg 
and  contemplations  men  spend  their  labour  in  investigating 
and  treating  of  the  first  principles  of  tilings,  and  the  extreme 
hmits  of  nature,  when  all  that  is  useful  and  of  avail  in 
operation  is  to  be  found  in  what  is  intermediate.  Hence  it 
happens  that  men  continue  to  absti-act  ISTature  till  they  arrive 
at  potential  and  unformed  matter ;  and  again  they  continue 
to  divide  l^ature,  until  they  have  arrived  at  the  atom ;  things 
which,  even  if  true,  can  be  of  little  use  in  helping  on  the 
fortunes  of  men." 

Surely  we  ought  to  learn  a  lesson  from  these  remarks 
of  the  great  Father  of  experimental  science,  and  be  very 
cautious  before  we  dismiss  any  branch  of  knowledge  or 
train  of  thought  as  essentially  unprofitable. 

Aristotle  on  a  Medium. 

18G.  As  regards  the  existence  of  a  medium,  it  is  re- 
marked by  Whewell  that  the  ancients  also  caught  a  glimpse 
of  the  idea  of  a  medium,  by  which  the  qualities  of  bodies, 
as  colovu-s  and  sounds  are  perceived,  and  he  quotes  the 
following  from  Ai'istotle  : — 

"  In  a  void  there  coidd  be  no  difference  of  up  and  do-svn ; 
for,  as  in  nothing  there  are  no  differences,  so  there  are  none 
in  a  privation  or  negation." 

Upon  this  the  historian  of  science  remarks,  "  It  is 
easily  seen  that  such  a  mode  of  reasoning  elevates 
the  familiar  forms  of  language,  and  the  intellectual  con- 
nexions of  terms,  to  a  supremacy  over  facts." 

Nevertheless,  may  it  not  be  replied  that  our  conceptions 


HISTORICAL   SKETCH.  135 

of  matter  are  deduced  from  the  familiar  experience,  that 
certain  portions  of  space  affect  us  in  a  certain  manner ; 
and,  consequently,  are  we  not  entitled  to  say  there  must 
be  something  wliere  we  experience  the  difference  of  up 
or  down  ?  Is  there,  after  all,  a  very  great  difference 
between  this  argument  and  that  of  modern  physicists  in 
favour  of  a  plenum,  who  tell  us  that  matter  cannot  act 
where  it  is  not  ? 

Aristotle  seems  also  to  have  entertained  the  idea  that 
light  is  not  any  body,  or  the  emanation  of  any  body  (for 
that,  he  says,  would  be  a  kind  of  body),  and  that  there- 
fore liglit  is  an  energy  or  act. 

The  Ideas  of  the  Ancients  tvcre  not  ProUJic. 

187.  These  quotations  render  it  evident  that  the 
ancients  had,  in  some  way,  grasped  the  idea  of  the 
essential  unrest  and  energy  of  things.  They  had  also  the 
idea  of  small  particles  or  atoms,  and,  finally,  of  a  medium 
of  some  sort.  And  yet  these  ideas  were  not  prolific — 
they  gave  rise  to  nothing  new. 

Now,  while  the  historian  of  science  is  unquestionably 
right  in  his  criticism  of  the  ancients,  that  their  ideas 
were  not  distinct  and  appropriate  to  the  facts,  yet  we 
have  seen  that  they  were  not  wholly  ignorant  of  the 
most  profound  and  deeply-seated  principles  of  the  mate- 
rial universe.  In  the  great  hymn  chanted  by  Nature,  the 
fundamental  notes  were  early  heard,  but  yet  it  reqidred 
long  centuries  of  patient  waiting  for  the  practised  ear  of 


i;-{6  THE   CONSERVATION   OF   ENERGY. 

tlie  skilled  musician  to  appreciate  the  might}'  hainion}' 
aright.  Or,  perhaps,  the  attempts  of  the  ancients  were 
as  the  sketches  of  a  child  who  just  contrives  to  ex- 
hibit, in  a  rude  way,  the  leading  outlines  of  a  building  ; 
while  tlie  conceptions  of  the  practised  physicist  are  more 
allied  to  those  of  the  architect,  or,  at  least,  of  one  who 
has  realized,  to  some  extent,  the  architect's  views. 

188.  The  ancients  possessed  great  genius  and  intellectual 
power,  but  they  were  deficient  in  physical  conceptions, 
and,  in  consequence,  their  ideas  were  not  prolific.  It 
cannot  indeed  be  said  that  we  of  the  present  age  are 
deficient  in  such  concej^tions ;  nevertheless,  it  may  be 
questioned  whether  there  is  not  a  tendency  to  rush  into 
the  opposite  extreme,  and  to  work  physical  conceptions  to 
an  excess.  Let  us  be  cautious  that  in  avoiding  Scylla,  we 
do  not  rush  into  Charybdis.  For  the  universe  has  more 
than  one  point  of  view,  and  there  are  possibly  regions 
which  will  not  yield  their  treasures  to  the  most  deter- 
mined ph3^sicists,  armed  only  with  kilogrammes  and 
metres  and  standard  clocks. 

Descartes,  Nevjton,  and  Huygkens  on  a  Medium. 

189.  In  modern  times  Descartes,  author  of  the  vortical 
hypothesis,  necessarily  presupposed  the  existence  of  a 
medium  in  inter-planetary  spaces,  but  on  the  other  hand 
he  was  one  of  the  originators  of  that  idea  which  regards 
light  as  a  series  of  particles  shot  out  from  a  luminous 
body.       Newton   likewise  conceived  the  existence  of  a 


HISTORICAL   SKETCH.  137 

medium,  although  he  became  an  advocate  of  the  theory  of 
emission.  It  is  to  Huyghens  that  the  credit  belongs  of 
having  first  conceived  the  undulatory  thcoiy  of  light 
with  sufficient  distinctness  to  account  for  double  refrac- 
tion. After  him,  Young,  Fresnel,  and  their  followers, 
hav«  greatly  developed  the  theory,  enabling  it  to  account 
for  the  most  complicated  and  wonderful  phenomena. 

Bacon  on  Heat. 

190.  With  regard  to  the  nature  of  heat,  Bacon,  what- 
ever may  be  thought  of  his  arguments,  seems  clearly  to 
have  recognized  it  as  a  species  of  motion.  He  says, 
"  From  these  instances,  viewed  together  and  individually, 
the  nature  of  which  heat  is  the  limitation  seems  to  be 
motion ; "  and  again  he  says,  "  But  when  we  say  of 
motion  that  it  stands  in  the  place  of  a  genus  to  heat,  we 
mean  to  convey,  not  that  heat  generates  motion  or  motion 
heat  (although  even  both  may  be  true  in  some  cases),  but 
that  essential  heat  is  motion  and  notliino:  else." 

Nevertheless  it  required  nearly  three  centuries  before 
the  true  theory  of  heat  was  sufficiently  rooted  to  develop 
into  a  productive  hypothesis. 

Principle  of  Virtual  Velocities. 

191.  In  a  previous  chapter  we  have  already  detailed 
the  labours  in  respect  of  heat  of  Davy,  Rumford,  and 
Joule.  Galileo  and  Newton,  if  they  did  not  gi'asp  the 
dynamical  nature  of  heat,  had  yet  a  clear  conception  of 


133  THE   CONSERVATION   OF    ENERGY. 

the  functions  of  a  machine.  The  former  saw  that  what 
we  gain  in  power  we  lose  in  space ;  while  the  latter  went 
further,  and  saw  that  a  machine,  if  left  to  itself,  is  strictly 
limited  in  the  amount  of  work  which  it  can  accomplish, 
although  its  energy  may  vary  from  that  of  motion  to 
that  of  position,  and  back  again,  according  to  the 
geometric  laws  of  the  machine. 

Rise  of  true  Conceptions  regarding  Work. 

192.  There  can,  we  think,  be  no  question  that  the  great 
development  of  industrial  operations  in  the  present  age 
has  indirectly  furthered  our  conceptions  regarding  work. 
Humanity  invariably  strives  to  escape  as  much  as 
possible  from  hard  work.  In  the  days  of  old  those 
who  had  the  power  got  slaves  to  work  for  them ; 
but  even  then  the  master  had  to  give  some  kind  of 
equivalent  for  the  work  done.  For  at  the  very  lowest  a 
slave  is  a  machine,  and  must  be  fed,  and  is  moreover  apt 
to  prove  a  very  troublesome  machine-  if  not  properly 
dealt  with.  The  great  improvements  in  the  steam 
engine,  introduced  by  Watt,  have  done  as  much,  perhaps, 
as  the  abolition  of  slavery  to  benej&t  the  working  man. 
The  hard  work  of  the  world  has  been  put  upon  iron 
shoulders,  that  do  not  smart;  and,  in  consequence,  we  have 
had  an  immense  extensicn  of  industry,  and  a  great 
amelioration  in  the  position  of  the  lower  classes  of  man- 
kind. But  if  we  have  transferred  our  hard  work  to 
macliines,  it  is  necessary  to   know  how  to  question  a 


HISTORICAL   SKETCH.  139 

machine — how  to  say  to  it,  At  what  rate  can  you 
labour  ?  how  much  work  can  you  turn  out  in  a  day  ? 
It  is  necessary,  in  fact,  to  have  the  clearest  possible  idea 
of  what  work  is. 

Our  readers  will  see  from  all  this  that  men  are  not 
likely  to  err  in  their  method  of  measuring  work.  The 
principles  of  measurement  have  been  stamped  as  it  were 
with  a  brand  into  the  very  heart  and  brain  of  humanity^ 
To  the  em][)loyer  of  machinery  or  of  human  labour,  a 
false  method  of  measuring  work  simply  means  ruin ;  he 
is  likely,  therefore,  to  take  the  greatest  possible  pains  to 
arrive  at  accuracy  in  his  determination. 

Perpetual  Motion. 

193.  Now,  amid  the  crowd  of  workers  smarting  from 
the  curse  of  labour,  there  rises  up  every  now  and  then 
an  enthusiast,  who  seeks  to  escape  by  means  of  an  artifice 
from  this  insupportable  tyranny  of  work.  Why  not 
construct  a  machine  that  will  go  on  giving  you  work 
without  limit  without  the  necessity  of  being  fed  in  any 
way.  Nature  must  have  some  weak  point  in  her  armour ; 
there  must  surely  be  some  way  of  getting  round  her ;  she 
is  only  tyrannous  on  the  surface,  and  in  order  to  stimulate 
our  ingenuity,  but  will  yield  with  pleasure  to  the  per- 
sistence of  genius. 

Now,  what  can  the  man  of  science  say  to  such  an 
enthusiast  ?  He  cannot  tell  him  that  he  is  intimately 
accjuainted  with  all  the  forces  of  Nature,  and  can  prove 


140  THE   CONSERVATION   OF   ENEEGY. 

that  perpetual  motion  is  impossible;  for,  in  truth,  he 
knows  very  little  of  these  forces.  But  he  does  think 
that  he  has  entered  into  the  spirit  and  design  of  Nature, 
and  therefore  he  denies  at  once  the  possibility  of  such 
a  machine.  But  he  denies  it  intelligently,  and  works 
out  this  denial  of  his  into  a  theory  which  enables  him 
to  discover  numerous  and  valuable  relations  between  the 
properties  of  matter — produces,  in  fact,  the  laws  of  energy 
and  the  great  principle  of  conservation. 

Theory  of  Conservation. 

194.  We  have  thus  endeavoured  to  give  a  short  sketch 
of  the  history  of  energy,  including  its  allied  problems,  up 
to  the  dawn  of  the  strictly  scientific  period.  We  have 
seen  that  the  unfruitfulness  of  the  earlier  views  was  due 
to  a  want  of  scientijfic  clearness  in  the  conceptions  enter- 
tained, and  we  have  now  to  say  a  few  words  regarding 
the  theory  of  conservation. 

Here  also  the  way  was  pointed  out  by  two  philoso- 
phers, namely,  Grove  in  this  country,  and  Mayer  on 
the  continent,  who  showed  certain  rela,tions  between 
the  various  forms  of  energy;  the  name  of  Seguin 
ought  likewise  to  be  mentioned.  Nevertheless,  to 
Joule  belongs  the  honour  of  establishing  the  theory  on 
an  incontrovertible  basis  :  for,  indeed,  this  is  pre- 
eminently a  case  where  speculation  has  to  be  tested  by 
unimpeachable  experimental  evidence.  Here  the  magni- 
tude of  the  principle  is  so  vast,  and  its  importance  is  so 


THE  DISSIPATION   OF   ENERGY.  141 

gi-eat,  that  it  requires  the  strong  fire  of  genius,  joined  to 
the  patient  labours  of  the  scientific  experimentalist,  to 
forge  the  rough  ore  into  a  good  weapon  that  will  cleave 
its  way  through  all  obstacles  into  the  very  citadel  of 
Nature,  and  into  her  most  secret  recesses. 

Following  closely  upon  the  labours  of  Joule,  we  have 
those  of  William  and  James  Thomson,  Helmholtz,  Ean- 
kine,  Clausius,  Tait,  Andrews,  Maxwell,  who,  along 
with  many  others,  have  advanced  the  subject ;  and  while 
Joule  gave  his  chief  attention  to  the  laws  which  regu- 
late the  transmutation  of  mechanical  energy  into  heat, 
Thomson,  Rankine,  and  Clausius  gave  theirs  to  the  con- 
verse problem,  or  that  which  relates  to  the  transmutation 
of  heat  into  mechanical  energy.  Thomson,  especially, 
has  pushed  forward  so  resolutely  from  this  point  of  view 
that  he  has  succeeded  in  grasping  a  principle  scarcely 
inferior  in  importance  to  that  of  the  conservation  of 
energy  itself,  and  of  this  principle  it  behoves  us  now  to 

speak. 

Dissipation  of  Energy. 

195.  Joule,  we  have  said,  proved  the  law  according 
to  which  work  may  be  changed  into  heat ;  and  Thomson 
and  others,  that  according  to  which  heat  may  be  changed 
into  work.  Now,  it  occurred  to  Thomson  that  there  was 
a  very  important  and  significant  difference  between  these 
two  laws,  consisting  in  the  fact  that,  while  you  can  with 
the  greatest  ease  transform  work  into  heat,  you  can  by 
110  method  in  your  power  transform  all  the  heat  back 


14:2  THE   CONSERVATION   OF   ENERGY. 

again  into  work.  In  fact,  the  process  is  not  a  reversible 
one ;  and  the  consequence  is  that  the  mechanical  energy 
of  the  universe  is  becoming  every  day  more  and  more 
changed  into  heat. 

It  is  easily  seen  that  if  the  process  were  reversible, 
one  form  of  a  perj)etual  motion  would  not  be  impossi- 
ble. For,  without  attempting  to  create  energy  by  a 
machine,  all  that  would  be  needed  for  a  perpetual  motion 
would  be  the  means  of  utilizing  the  vast  stores  of  heat 
that  lie  in  all  the  substances  around  us,  and  convertins 
them  into  work.  The  work  would  no  doubt,  by  means 
of  friction  and  otherwise,  be  ultimately  reconverted  into 
heat ;  but  if  the  process  be  reversible,  the  heat  could 
again  be  converted  into  work,  and  so  on  for  ever.  But 
the  irreversibility  of  the  process  puts  a  stop  to  all  this. 
In  fact,  I  may  convince  myself  by  rubbing  a  metal 
button  on  a  piece  of  wood  how  easily  work  can  be 
converted  into  heat,  while  the  mind  completely  fails  to 
suggest  any  method  by  which  this  heat  can  be  recon- 
verted into  work. 

Now,  if  this  process  goes  on,  and  always  in  one 
direction,  there  can  be  no  doubt  about  the  issue.  The 
mechanical  energy  of  the  universe  will  be  more  and 
more  transformed  into  universally  diffused  heat,  until  the 
universe  wiU  no  longer  be  a  fit  abode  for  living  beings. 

The  conclusion  is  a  startling  one,  and,  in  order  to 
bring  it  more  vividly  before  our  readers,  let  us  now  pro- 
ceed to  acquaint  ourselves  with  the  various  forms  of  use- 


THE  DISSIPATION   OF   ENERGY.  143 

ful  energy  that  are  at  present  at  our  disposal,  and  at  tlie 
same  time  endeavour  to  trace  the  ultimate  sources  of 
these  supplies. 

Natural  Energiea  and  their  Sources. 

196.  Of  energy  in  repose  we  have  the  following 
varieties  : — (1.)  The  energy  of  fuel.  (2.)  That  of  food. 
(3.)  That  of  a  head  of  water.  (4.)  That  which  may  be 
derived  from  the  tides.  (5.)  The  energy  of  chemical 
separation  implied  in  native  sulphur,  native  iron,  &c. 

Then,  with  regard  to  energy  in  action,  we  have  mainly 
the  following  varieties  : — 

(1.)  The  energy  of  air  in  motion.  (2.)  That  of  water 
in  motion. 

Fuel 

197.  Let  us  begin  first  with  the  energy  implied  in  fuel. 
We  can,  of  course,  burn  fuel,  or  cause  it  to  combine  with 
the  oxygen  of  the  air  ;  and  we  are  thereby  provided  with 
large  quantities  of  heat  of  high  temperature,  by  means  of 
wliich  we  may  not  only  warm  ourselves  and  cook  our 
food,  but  also  drive  our  heat-engines,  using  it,  in  fact,  as 
a  source  of  mechanical  power. 

Fuel  is  of  two  varieties — wood  and  coaL  Now,  if  we 
consider  the  origin  of  these  we  shall  see  that  they  are 
produced  by  the  sun's  rays.  Certain  of  these  rays, 
as  we  have  already  remarked  (Art.  180),  decompose 
carbonic  acid  in  the  leaves  of  plants,  setting  free  tho 


U-k  THE   CONSERVATION   OF   ENERGY. 

oxygen,  while  the  carbon  is  used  for  the  structure  or 
wood  of  the  plant.  Now,  the  energy  of  these  ra}'s  is 
spent  in  this  process,  and,  indeed,  there  is  not  enough 
of  such  energy  left  to  produce  a  good  photograpliic  im- 
pression of  the  leaf  of  a  plant,  because  it  is  all  spent  in 
making  wood. 

We  thus  see  that  the  energy  implied  in  wood  is 
derived  from  the  sun's  rays,  and  the  same  remark  applies 
to  coal.  Indeed,  the  only  difference  between  wood  and 
coal  is  one  of  age :  wood  being  recently  turned  out  from 
Nature's  laboratory,  while  thousands  of  years  have  elapsed 
since  coal  formed  the  leaves  of  living  plants. 

198.  We  are,  therefore,  perfectly  justified  in  saying  that 
the  energy  of  fuel  is  derived  from  the  sun's  rays ;  *  coal 
being  the  store  which  Nature  has  laid  up  as  a  species  of 
capital  for  us,  while  wood  is  our  precarious  yearly  income. 

We  are  thus  at  present  very  much  in  the  position 
of  a  young  heir,  who  has  only  recently  come  into  his 
estate,  and  who,  not  content  with  the  income,  is  rapidly 
squandering  his  realized  property.  This  subject  has  been 
forcibly  brought  before  us  by  Professor  Jevons,  who 
has  remarked  that  not  only  are  we  spending  our 
capital,  but  we  are  spending  the  most  available  and 
valuable  part  of  it.  For  we  are  now  using  the  surface 
coal ;  but  a  time  will  come  when  this  will  be  exhausted, 
and  we  shall  be  compelled  to   go  deep  down  for  our 

•  This  fact  seems  to  have  been  known  at  a  comparatively  'early  period 
to  Herscliel  and  the  elder  Stephenson. 


TUE   DISSIPATION   OF   ENERGY.  1-45 

supplies.  Now,  regarded  as  a  source  of  energy,  sucli 
supplies,  if  far  down,  will  be  less  effective,  for  we  have 
to  deduct  the  amount  of  energy  requisite  in  order  to 
bring  them  to  tlie  surface.  The  result  is  that  we  must 
contemplate  a  time,  however  far  distant,  when  our  su])- 
plies  of  coal  will  be  exhausted,  and  we  shall  be  com- 
pelled to  resort  to  other  sources  of  energy. 

Food. 

199.  The  energy  of  food  is  analogous  to  that  of  fuel, 
and  serves  similar  purposes.  For  just  as  fuel  may  be 
used  either  for  producing  heat  or  for  doing  work,  so  food 
has  a  twofold  office  to  perform.  In  the  first  place,  by  its 
gradual  oxidation,  it  keeps  up  the  temperature  of  the 
body;  and  in  the  next  place  it  is  used  as  a  source  of 
energy,  on  which  to  draw  for  the  performance  of  work. 
Thus  a  man  or  a  horse  that  works  a  great  deal  requires 
to  eat  more  food  than  if  he  does  not  work  at  all  Thus, 
also,  a  prisoner  condemned  to  hard  labour  requires  a 
better  diet  than  one  who  does  not  work,  and  a  soldier 
during  the  fatigues  of  war  finds  it  necessary  to  eat  more 
than  during  a  time  of  peace. 

Our  food  may  be  either  of  animal  or  vegetable  origin — 
if  it  be  the  latter,  it  is  immediately  derived,  like  fuel, 
from  the  energy  of  the  sun's  rays ;  but  if  it  be  the  former, 
the  only  difference  is  that  it  has  passed  through  the  body 
of  an  animal  before  coming  to  us :  the  animal  has  eaten 
gi-ass,  and  we  have  eaten  the  animal. 


146  THE   CONSERVATION   OF   ENERCY. 

In  fact,  we  make  use  of  the  animal  not  only  as  a 
variety  of  nutritious  food,  but  also  to  enable  us  indirectly 
to  utilize  those  vegetable  products,  such  as  grasses,  which 
we  could  not  make  use  of  directly  with  our  present 
digestive  organs. 

Head  of  Water. 

200.  The  energy  of  a  head  of  water,  like  that  of  fuel 
and  food,  is  brought  about  by  the  sun's  rays.  For  the 
sun  vaporizes  the  water,  which,  condensed  again  in  up- 
land distiicts,  becomes  available  as  a  head  of  water. 

There  is,  however,  the  difference  that  fuel  and  food  arc 
due  to  the  actinic  power  of  the  sun's  rays,  while  the 
evaporation  and  condensation  of  water  are  caused  rather 
by  their  heating  effect. 

Tidal  Energy. 

201.  The  energy  derived  from  the  tides  has,  however, 
a  different  origin.  In  Art.  133  we  have  endeavoured  to 
show  how  the  moon  acts  upon  the  fluid  portions  of 
our  globe,  the  result  of  this  action  being  a  very  gradual 
stoppage  of  the  energy  of  rotation  of  the  earth. 

It  is,  therefore,  to  this  motion  of  rotation  that  we 
must  look  as  the  origin  of  any  available  energy  derived 
from  tidal  mills. 


THE   DISSIPATION   OF   ENErvGY.  l-i7 

Kative  Sulj^hur,  etc. 

202.  The  last  variety  of  available  energy  of  position 
hi  our  list  is  tliat  implied  in  native  sulphur,  native  iron, 
fcc.  It  lias  been  remarked  by  Professor  Tait,  to  whom 
this  method  of  reviewing  our  forces  is  due,  that  this  may 
be  the  primeval  form  of  energy,  and  that  the  interior  of 
the  earth  may,  as  far  as  we  know,  be  wholly  composed  of 
matter  in  its  uncombined  form.  As  a  sonrce  of  available 
energy  it  is,  however,  of  no  practical  importance. 

Air  and  Water  in  Motion. 

203.  We  proceed  next  to  those  varieties  of  available 
energy  which  represent  motion,  the  chief  of  which  are 
air  in  motion  and  water  in  motion.  It  is  owing  to  the 
former  that  the  mariner  spreads  liis  sail,  and  carries  his 
vessel  from  one  part  of  the  earth's  siu'face  to  another, 
and  it  is  likewise  owing  to  the  same  influence  that  the 
windmill  grinds  our  corn.  Again,  water  in  motion  is 
used  perhaps  even  more  frequently  than  air  in  motion  as 
a  source  of  motive  power. 

Both  these  varieties  of  energy  are  due  without  doubt 
to  the  heating  effect  of  the  sun's  rays.  "We  may,  there- 
fore, affirm  that  with  the  exception  of  the  totally  insig- 
nificant supply  of  native  sulphur,  &c.,  and  the  small 
number  of  tidal  mills  which  may  be  in  operation,  all 
our  available  energy  is  due  to  the  sun. 


143  TliE   CONSEnVATION   OF   ENERGY. 

Tlic  Sun — a  Source  of  High  Temperature  Heat 

204<.  Let  U3,  therefore,  now  for  a  moment  direct  our 
attention  to  that  most  wonderful  source  of  energy,  the 
Sun. 

We  have  here  a  vast  reservoir  of  high  temperature 
heat ;  now,  this  is  a  kind  of  superior  energy  which  has 
always  been  in  much  request.  Numberless  attempts 
have  been  made  to  construct  a  perpetual  light,  just  as 
similar  attempts  have  been  made  to  construct  a  perpetual 
motion,  with  this  difference,  that  a  perpetual  light  was 
supposed  to  result  from  magical  powers,  while  a  perpetual 
motion  was  attributed  to  mechanical  skill. 

Sir  Walter  Scott  alludes  to  this  belief  in  his  de- 
scription of  the  grave  of  Michael  Scott,  which  is  made 
to  contain  a  perpetual  light.  Thus  the  Monk  who  buried 
the  wizard  tells  William  of  Deloraine — • 

"  Lo,  Warrior  !  now  tlie  Cross  of  Eed 
Points  to  the  Grave  of  the  mighty  dead; 
Within  it  burns  a  wondrous  light, 
To  chase  the  spirits  that  love  the  night. 
That  lamp  shall  bum  unquenchably 
Until  the  eternal  doom  shall  be." 

Mid  again,  when  the  tomb  was  opened,  we  i-ead — 

"  I  would  you  had  been  there  to  see 

How  the  light  broke  forth  so  gloriously, 
Stream'd  upward  to  the  chancel  roof, 
And  through  the  galleries  far  aloof  ! 
No  earthly  flame  blazed  e'er  so  bright," 


THE   DISSIPATION   OF   ENERGY.  149 

No  earthly  flame — there  the  poet  was  right — certainly 
not  of  this  earth,  where  light  and  all  other  forms  of 
superior  energy  are  essentially  evanescent. 

A  Peiyetual  Light  Impossible. 

205.  In  truth,  our  readers  will  at  once  perceive  that 
a  perpetual  light  is  only  another  name  for  a  perpetual 
motion,  because  we  can  always  derive  visible  energy  out 
of  high  temperature  heat — indeed,  we  do  so  every  day 
in  our  steam  engines. 

When,  therefore,  we  burn  coal,  and  cause  it  to  combine 
with  the  oxygen'  of  the  air,  we  derive  from  the  process  a 
large  amount  of  high  temperature  heat.  But  is  it  not 
possible,  our  readers  may  ask,  to  take  the  carbonic  acid 
which  results  from  the  combustion,  and  by  means  of  low 
temperature  heat,  of  which  we  have  always  abundance  at 
our  disposal,  change  it  back  again  into  carbon  and  oxygen  ? 
All  this  would  be  possible  if  what  may  be  termed  the 
temperature  of  disassociation  —  that  is  to  say,  the 
temperature  at  which  carbonic  acid  separates  into  its 
constituents — were  a  low  temperature,  and  it  would  also 
be  possible  if  rays  from  a  source  of  low  temperature  pos- 
sessed sufficient  actinic  power  to  decompose  carbonic  acid. 

But  neither  of  these  is  the  case.  Nature  will  not  be 
caught  in  a  trap  of  this  kind.  As  if  for  the  very  pur- 
pose of  stopping  all  such  speculations,  the  temperatures 
of  disassociation  for  such  substances  as  carbonic  acid  are 
very  high,  and  the  actinic  rays  capable  of  causing  their 


150  THE   CONSERVATION   OF  ENERGY. 

decomposition  belong  only  to  sources  of  exceedingly  high 
temperature,  such  as  the  sun.* 

Is  the  Sun  an  Exception? 

206.  We  may,  therefore,  take  it  for  granted  that  a  per- 
petual light,  like  a  perpetual  motion,  is  an  impossibility ; 
and  we  have  then  to  inquire  if  the  same  argument 
applies  to  our  sun,  or  if  an  exception  is  to  be  made  in 
his  favour.  Does  the  sun  stand  upon  a  footing  of  his 
own,  or  is  it  merely  a  question  of  time  with  him,  as  with 
all  other  instances  of  high  temperature  heat  ?  Before 
attempting  to  answer  this  question  let  us  inquire  into  the 
probable  origin  of  the  sun's  heat. 

Origin  of  the  Sun's  Heat. 

207.  Now,  some  might  be  disposed  to  cut  the  Gordiaii 
knot  of  such  an  inquiry  by  asserting  that  our  luminary 
was  at  first  created  hot ;  yet  the  scientific  mind  finds 
itself  disinclined  to  repose  upon  such  an  assertion.  We 
p'ck  up  a  round  pebble  from  the  beach,  and  at  once 
acknowledge  there  has  been  some  physical  cause  for  the 
shape  into  which  it  has  been  worn.  And  so  with  regard 
to  the  heat  of  the  sun,  we  must  ask  ourselves  if  there 
be  not  some  cause  not  wholly  imaginary,  but  one  which 
we  know,  or  at  least  suspect,  to  be  perhaps  still  in  opera- 
tion, wliich  can  account  for  the  heat  of  the  sun. 

Now,  here   it  is   more   easy    to    show   what   cannot 

*  Tliis  remark  is  due  to  Sir  William  Thomson. 


TUE   DISSIPATION   OF   ENERGY.  151 

account  for  the  sun's  heat  than  what  can  do  so.  We 
may,  for  instance,  be  perfectly  certain  that  it  cannot 
have  been  caused  by  chemical  action.  The  most  probable 
theory  is  that  which  was  first  worked  out  by  Helmholtz 
and  Thomson ;  *  and  which  attributes  the  heat  of  the 
sun  to  the  primeval  energy  of  position  possessed  by  its 
particles.  In  other  words,  it  is  supposed  that  these  parti- 
cles originally  existed  at  a  great  distance  from  each  other, 
and  that,  being  endowed  with  the  force  of  gravitation,  they 
have  since  gradually  come  together,  while  in  this  process 
heat  has  been  generated  just  as  it  would  be  if  a  stone  were 
dropped  from  the  top  of  a  cliff  towards  the  earth. 

208.  Nor  is  this  case  wholly  imaginary,  but  we  have 
some  reason  for  thinking  that  it  may  still  be  in  operation 
in  the  case  of  certain  nebulse  which,  both  in  their  consti- 
tution as  revealed  by  the  spectroscope,  and  in  their 
general  appearance,  impress  the  beholder  with  the  idea 
that  they  are  not  yet  fully  condensed  into  their  ultimate 
shape  and  size. 

If  we  allow  that  by  this  means  our  luminary  has 
obtained  his  wonderful  store  of  high-class  energy,  we 
have  yet  to  inquire  to  what  extent  this  operation  is 
going  on  at  the  present  moment.  Is  it  only  a  thing 
of  the  past,  or  is  it  a  thing  also  of  the  present  ?  I 
think  we  may  reply  that  the  sun  cannot  be  condensing 
very  fast,  at  least,  within  historical  times.      For  if  the 

•  Mayer  and  Waterston  seem  first  to  have  caught  the  rudiments  of 
fhis  idea. 


L52  THE   CONSERVATION   OF  ENERGY. 

sun  were  sensibly  larger  than  at  present  liis  total  eclipse 
by  the  moon  would  be  impossible.  Now,  such  eclipses 
have  taken  place,  at  any  rate,  for  several  thousands  of  years. 
Doubtless  a  small  army  of  meteors  may  be  falling  into 
our  luminary,  which  would  bj^-  this  fall  tend  to  augment 
his  heat ;  yet  the  supply  derived  from  this  source  must 
surely  be  insignificant.  But  if  the  sun  be  not  at  present 
condensing  so  fast  as  to  derive  any  sufficient  heat  from  this 
process,  and  if  his  energy  be  very  sparingly  recruited 
from  without,  it  necessarily  follows  that  he  is  in  the 
position  of  a  man  whose  expenditure  exceeds  his  income. 
He  is  living  upon  his  capital,  and  is  destined  to  share  the 
fate  of  all  who  act  in  a  similar  manner.  We  must,  there- 
fore, contemplate  a  future  period  when  he  will  be  poorer 
in  energy  than  lie  is  at  present,  and  a  period  still  further 
in  the  future  when  he  will  altogether  cease  to  shine. 

Prohahle  Fate  of  the  Universe. 

209.  If  this  be  the  fate  of  the  high  temperature 
energy  of  the  universe,  let  us  think  for  a  moment  w^hat 
will  happen  to  its  visible  energy.  We  have  spoken 
already  about  a  medium  pervading  space,  the  office  of 
which  appears  to  be  to  degrade  and  ultimately  extinguish 
all  differential  motion,  just  as  it  tends  to  reduce  and  ulti- 
mately equalize  all  difference  of  temperature.  Thus  the 
universe  would  ultimately  become  an  equally  heated 
mass,  utterly  worthless  as  far  as  the  production  of  woik 
is  concerned,  since  such  production  depends  uj)on  differ- 
ence of  tempei'ature. 


THE  DISSIPATION^    OF   ENERGY.  153 

Although,  therefore,  in  a  strictly  mechanical  sense, 
there  is  a  conservation  of  energy,  yet,  as  regards  use- 
fulness or  fitness  for  living  beings,  the  energy  of  the 
universe  is  in  process  of  deterioration.  Universally 
difiused  heat  forms  what  we  may  call  the  great  waste- 
heap  of  the  universe,  and  this  is  gi-owing  larger  year 
by  year.  At  present  it  does  not  sensibly  obtrude  itself, 
but  who  knows  that  the  time  may  not  arrive  when  wo 
shaU  be  practically  conscious  of  its  growing  bigness  ? 

210.  It  will  be  seen  that  in  this  chapter  we  have  re- 
garded the  universe,  not  as  a  collection  of  matter,  but 
rather  as  an  energetic  agent — in  fact,  as  a  lamp.  Now,  it 
has  been  well  pointed  out  by  Thomson,  that  looked  at  in 
this  light,  the  universe  is  a  system  that  had  a  beginning 
and  must  have  an  end;  for  a  process  of  degradation 
cannot  be  eternal.  If  we  could  view  the  universe  as  a 
candle  not  lit,  then  it  is  perhaps  conceivable  to  regard  it 
as  having  been  always  in  existence ;  but  if  we  regard  it 
rather  as  a  candle  that  has  been  lit,  we  become  absolutely 
certain  that  it  cannot  have  been  burning  from  eternity, 
and  that  a  time  wiU  come  when  it  will  cease  to  burn. 
We  are  led  to  look  to  a  beginning  in  which  the  particles 
of  matter  were  in  a  diffuse  chaotic  state,  but  endowed 
with  the  power  of  gravitation,  and  we  are  led  to  look  to 
an  end  in  which  the  whole  universe  will  be  one  equally 
heated  inert  mass,  and  from  which  everything  like  life  or 
motion  or  beauty  will  have  utterly  gone  away. 


L54<  TILE  C0:TSEr.YATION   OF  EJS^EEGY. 


CHAPTER.  YL 

THE  POSITION  OF  LIFE. 

211.  Y\''e  have  Litlierto  confined  ourijelves  almost 
entii'ely  to  a  discussion  of  the  laws  of  energy,  as  these 
affect  inanimate  matter,  and  have  taken  little  or  no  account 
of  the  position  of  life.  We  have  been  content  very  much 
to  remain  spectators  of  the  contest,  apparently  forgetful 
that  we  are  at  all  concerned  in  the  issue.  But  the  con- 
flict is  not  one  which  admits  of  on-lookers, — it  is  a  uni- 
versal conflict  in  which  we  must  all  take  our  share.  It 
may  not,  therefore,  be  amiss  if  we  endeavour  to  ascertain, 
as  well  as  we  can,  our  true  position. 

Twofold  nature  of  Equilihrium. 

212.  One  of  our  earliest  mechanical  lessons  is  on  the 
twofold  nature  of  equilibrium.  We  are  told  that  this 
may  be  of  two  kinds,  stable  and  unstable,  and  a  very 
good  illustration  of  these  two  kinds  is  furnished  by  an 
egg.  Let  us  take  a  smooth  level  table,  and  place  an  egg 
upon  it ;  we  all  know  in  what  manner  the  egg  wiU  lie 


HIE   POSITION   OF   LIFE.  155 

OQ  the  table.  It  will  remain  at  rest,  that  is  to  say,  it 
will  be  in  equilibrium ;  and  not  only  so,  but  it  will  be  in 
stable  equilibrium.  To  prove  this,  let  us  tiy  to  displace 
it  with  our  finger,  and  we  shall  find  that  when  we  remove 
the  pressure  the  egg  will  speedily  return  to  its  previous 
position,  and  will  come  to  rest  after  one  or  two  oscilla- 
tions. Furthermore,  it  has  required  a  sensible  expenditure 
of  energy  to  displace  the  egg.  All  this  we  express  by 
saying  that  the  egg  is  in  stable  equilibrium. 

J\lech an ical  Jnsta hlli ty. 

213.  And  now  let  us  try  to  balance  the  egg  upon  its 
longer  axis.  Probably,  a  sufficient  amount  of  care  will 
enable  us  to  achieve  this  also.  But  the  operation  is  a 
difficult  one,  and  requires  great  delicacy  of  touch,  and  even 
after  we  have  succeeded  we  do  not  know  how  long  our 
success  may  last.  The  slightest  impulse  from  without,  the 
merest  breath  of  air,  may  be  sufficient  to  overturn  the 
egg,  which  is  now  most  evidently  in  unstable  equilibrium. 
If  the  egg  be  thus  balanced  at  the  very  edge  of  the  table, 
it  is  quite  probable  that  in  a  few  minutes  it  may  topple 
over  upon  the  fioor;  it  is  what  we  may  call  an  even 
chance  whether  it  will  do  so,  or  merely  fall  upon  the 
table.  Not  that  mere  chance  has  anything  to  do  with 
it,  or  that  its  movements  are  without  a  cause,  but  we 
mean  that  its  movements  are  decided  by  some  external 
impulse  so  exceedingly  small  as  to  be  utterly  beyond  our 
powers  of  observation.     In  fact,  before  making  the  trial 


L5G  THE   CONSERVATION  OF  ENERGY. 

we  have  carefully  removed  everytliing  like  a  current  oi 
air,  or  want  of  level,  or  external  impulse  of  any  kind, 
so  that  when  the  egg  falls  we  are  completely  unable  to 
assign  the  origin  of  the  impulse  that  has  caused  it  to 
do  so. 

214.  Now,  if  the  egg  happens  to  fall  over  the  table 
upon  the  floor,  there  is  a  somewhat  considerable  trans- 
mutation of  energy ;  for  the  energy  of  position  of  the  egg, 
due  to  the  height  which  it  occupied  on  the  table,  has  all 
at  once  been  changed  into  energy  of  motion,  in  the  first 
place,  and  into  heat  in  the  second,  when  the  egg  comes 
into  contact  with  the  floor. 

If,  however,  the  egg  happens  to  fall  upon  the  table,  the 
transmutation  of  energy  is  comparatively  small. 

It  thus  appears  that  it  depends  upon  some  external 
impulse,  so  infinitesimally  small  as  to  elude  our  observa- 
tion, whether  the  egg  shall  fall  upon  the  floor  and  give 
rise  to  a  comparatively  large  transmutation  of  energy,  or 
whether  it  shall  fall  upon  the  table  and  give  rise  to  a 
transmutation  comparatively  small. 

Chemical  Instability. 

215.  We  thus  see  that  a  body,  or  system,  in  unstable 
equilibrium  may  become  subject  to  a  very  considerable 
transmutation  of  energy,  arising  out  of  a  very  small 
cause,  or  antecedent.  In  the  case  now  mentioned,  the 
force  is  that  of  gravitation,  the  arrangement  being  one  of 
visible  mechanical  instability.     But  we  may  have  a  sub- 


THE   POSITION   OF   LIFE. 


157 


stance,  or  system,  in  which  the  force  at  work  is  not  gravity, 
but  chemical  affinity,  and  the  substance,  or  system,  may, 
under  certain  peculiar  conditions,  become  chemically 
unstable. 

When  a  substance  is  chemically  unstable,  it  means 
that  the  slightest  impulse  of  any  kind  may  determine 
a  chemical  change,  just  as  in  the  case  of  the  e^g  the 
slightest  impulse  from  without  occcasioned  a  mechanical 
displacement. 

In  fine,  a  substance,  or  system,  chemically  unstable 
bears  a  relation  to  chemical  affinity  somewhat  similar 
to  that  which  a  mechanically  unstable  system  bears 
to  gravity.  Gunpowder  is  a  familiar  instance  of 
a  chemically  unstable  substance.  Here  the  slightest 
spark  may  prove  the  precursor  of  a  sudden  chemical 
change,  accompanied  by  the  instantaneous  and  violent 
generation  of  a  vast  volume  of  heated  gas.  The  various 
explosive  compounds,  such  as  gun-cotton,  nitro-glycerine, 
the  fulminates,  and  many  more,  are  all  instances  of 
structures  which  are  chemically  unstable. 

Macldnes  are  of  two  kinds. 

216.  When  we  speak  of  a  structure,  or  a  machine,  or 
a  system,  we  simply  mean  a  number  of  individual  par- 
ticles associated  together  in  producing  some  definite 
result.  Thus,  the  solar  system,  a  timepiece,  a  rifle,  are 
examples  of  inanimate  machines ;  while  an  animal,  a 
human  being,  an  army,  are  examples  of  animated  struc- 


158  THE  CONSERVATION   OF  ENEIIGY. 

tures  or  machines.  Now,  sucli  macliines  or  structures 
are  of  two  kinds,  whicli  differ  from  one  another  not 
ordy  in  the  ohject  sought,  hut  also  in  the  means  of 
attaining  that  object. 

2]  7.  In  the  first  place,  we  have  structures  or 
machines  in  which  systematic  action  is  the  object  aimed 
at,  and  in  which  all  the  arrangements  are  of  a  conserva- 
tive nature,  the  element  of  instability  being  avoided  as 
much  as  possible.  The  solar  system,  a  timepiece,  a 
steam-engine  at  work,  are  examples  of  such  machines, 
and  the  characteristic  of  all  such  is  their  calculdbility. 
Thus  the  skilled  astronomer  can  tell,  with  the  utmost 
precision,  in  what  place  the  moon  or  the  planet  Venus 
will  be  found  this  time  next  year.  Or  again,  the 
excellence  of  a  timepiece  consists  in  its  various  hands 
pointing  accurately  in  a  certain  direction  after  a  certain 
interval  of  time.  In  like  manner  we  may  safely  count 
upon  a  steamship  making  so  many  knots  an  hour,  at 
least  while  the  outward  conditions  remain  the  same.  In 
all  these  cases  we  make  our  calculations,  and  we  are  not 
deceived — the  end  sought  is  regularity  of  action,  and  the ' 
means  employed  is  a  stable  arrangement  of  the  forces  of 
nature. 

218.  Now,  the  characteristics  of  the  other  class  of 
machines  are  precisely  the  reverse. 

Here  the  object  aimed  at  is  not  a  regular,  but  a  sudden 
and  violent  transmutation  of  energy,  while  the  means 
employed  are  unstable  arrangements  of  natural  forces. 


THE   POSITION   OF   LIFE.  159 

A.  rifle  at  full  cock,  with  a  delicate  Lair-tiigger,  is  a  very 
good  instance  of  such  a  machine,  where  the  slightest 
touch  from  without  may  bring  about  the  explosion  of  the 
gunpowder,  and  the  propulsion  of  the  ball  with  a  very  great 
velocity.  Now,  such  machines  are  eminently  characterized 
by  their  incalculability. 

219.  To  make  our  meaning  clear,  let  us  suppose  that 
two  sportsmen  go  out  hunting  together,  each  with  a 
good  rifle  and  a  good  pocket  chronometer.  After  a  hard 
day's  work,  the  one  turns  to  his  companion  and  says : — 
"  It  is  now  six  o'clock  by  my  watch ;  we  had  better  rest 
ourselves,"  upon  which  the  other  looks  at  his  w^atch,  and 
he  would  be  very  much  surprised  and  exceedingly 
indignant  with  the  maker,  if  he  did  not  find  it  six  o'clock 
also.  Their  chronometers  are  evidently  in  the  same  state, 
and  have  been  doing  the  same  thing;  but  what  about 
their  rifles  ?  Given  the  condition  of  the  one  rifle,  is  it 
possible  by  any  refinement  of  calculation  to  deduce  that 
of  the  other  ?  We  feel  at  once  that  the  bare  supposi- 
tion is  ridiculous. 

220.  It  is  thus  apparent  that,  as  regards  energy, 
.structures  are  of  two  kinds.  In  one  of  these,  the  object 
sought  is  regularity  of  action,  and  the  means  employed, 
a  stable  arrangement  of  natural  forces  :  while  in  the  other, 
the  end  sought  is  freedom  of  action,  and  a  sudden  trans- 
mutation of  energy,  the  means  employed  being  an  un- 
stable arrangement  of  natural  forces. 

The  one    set   of  machines  are   characterized  by  their 


IGO         THE  CONSERVATION  OF  ENERGT, 

calculability — the  other  by  their  incalculability.  Tlie 
one  set,  when  at  work,  are  not  easily  put  wrong,  while 
the  otlier  set  are  characterized  by  great  delicacy  of  coi)- 
struction. 

An  Animal  is  a  delicately-constructed  Machine. 

221.  But  perhaps  the  reader  may  object  to  our  use  of 
the  rifle  as  an  illustration. 

For  although  it  is  undoubtedly  a  delicately-constructed 
machine,  yet  a  rifle  does  not  represent  the  same  surpass- 
ing delicacy  as  that,  for  instance,  which  characterizes  an 
egg  balanced  on  its  longer  axis.  Even  if  at  full  cock, 
and  with  a  hair  trigger,  we  may  be  perfectly  certain  it 
will  not  go  off  of  its  own  accord.  Although  its  object  is 
to  produce  a  sudden  and  violent  transmutatioli  of  energy, 
yet  this  requires  to  be  preceded  by  the  application  of  an 
amount  of  energy,  however  small,  to  the  trigger,  and  if 
this  be  not  spent  upon  the  rifle,  it  "will  not  go  off.  There 
is,  no  doubt,  delicacy  of  construction,  but  this  has  not 
risen  to  the  height  of  incalculability,  and  it  is  only  when 
in  the  hands  of  the  sportsman  that  it  becomes  a  machine 
upon  the  condition  of  which  we  cannot  calculate. 

Now,  in  making  this  remark,  we  define  the  position 
of  the  sportsman  himself  in  the  Universe  of  Energy, 

The  rifle  is  delicately  constructed,  but  not  surpassingly 
so  ;  but  sportsman  and  rifle,  together,  form  a  machine 
of  surpassing  delicacy,  ergo  the  sportsman  himself  is 
such  a  machine.      We   thus   begin  to   perceive   that   ti 


THE   POSITION   OF   LIFE.  IGl 

human  being,  or  indeed  an  animal  of  any  kind,  is  in 
truth  a  machine  of  a  delicacy  that  is  practically  infinite, 
the  condition  or  motions  of  which  we  are  utterly  unable 
to  predict. 

In  truth,  is  there  not  a  transparent  absurdity  in  the 
very  thought  that  a  man  may  become  able  to  calculate 
his  own  movements,  or  even  those  of  his  fellow  ? 

Life  is  like  the  Comonander  of  an  Army 

222.  Let  us  now  introduce  another  analogy — let  us 
suppose  that  a  war  is  being  carried  on  by  a  vast  army, 
at  the  head  of  which  there  is  a  very  great  commander. 
Now,  this  commander  kiiows  too  well  to  expose  his  per- 
son ;  in  truth,  he  is  never  seen  by  any  of  his  subordinates. 
He  remains  at  work  in  a  well-guarded  room,  from  which 
telegraphic  wires  lead  to  the  headquarters  of  the  various 
divisions.  He  can  thus,  by  means  of  these  wires,  transmit 
his  orders  to  the  generals  of  these  divisions,  and  by  the 
same  means  receive  back  information  as  to  the  condition 
of  each. 

Thus  his  headquarters  become  a  centre,  into  which  all 
information  is  poured,  and  out  of  which  all  commands  are 
issued. 

Now,  that  mysterious  thing  called  life,  about  the  nature 
of  which  we  know  so  little,  is  probably  not  unlike  s\ich 
a  commander.  Life  is  not  a  bully,  who  swaggers  out 
into  the  open  universe,  upsetting  the  laws  of  energy  in 
all  directions,  but  rather  a  consummate  strategist,  who, 


162        THE  CONSERVATION  OF  ENERGY. 

sitting  in  his  secret  chamber,  before  his  wires,  directs  the 
movements  of  a  great  army.* 

223.  Let  us  next  suppose  that  our  imaginary  army  is 
in  rapid  march,  and  let  us  try  to  find  out  the  cause  of 
this  movement.  We  find  that,  in  the  first  place,  orders 
to  march  have  been  issued  to  the  troops  under  them  by 
the  commanders  of  each  regiment.  In  the  next  place,  we 
learn  that  staff  officers,  attached  to  the  generals  of  the 
various  divisions,  have  conveyed  these  orders  to  the 
regimental  commanders ;  and,  finally,  we  learn  that  the 
order  to  march  has  been  telegraphed  from  headquarters 
to  these  various  generals. 

Descending  now  to  ourselves,  it  is  probably  somewhere 
in  the  mysterious  and  well-guarded  brain-chamber  that 
the  delicate  directive  touch  is  given  which  determines 
our  movements.  This  chamber  forms,  as  it  were,  the 
headquarters  of  the  general  in  command,  who  is  so  well 
withdrawn  as  to  be  absolutely  invisible  to  all  his  sub- 
ordinates. 

22-i.  Joule,  Carpenter,  and  Mayer  were  at  an  early 
period  aware  of  the  restrictions  under  which  animals  are 
placed  by  the  laws  of  energy,  and  in  virtue  of  which  the 
power  of  an  animal,  as  far  as  energy  is  concerned,  is  not 
creative,  but  only  dii'ective.     It  was  seen  that,  in  order 

*  See  an  article  on  "  The  Position  of  Life,"  by  the  author  of  this 
work,  in  conjunction  with  Mr.  J.  N.  Lockyer,  "  Macmillan's  Magazine," 
September,  1868  ;  also  a  lecture  on  "  The  Recent  Developments  of  Cos- 
mical  Physics,"  by  the  author  of  this  work. 


THE   POSITION   OF   LIFE.  IGS 

to  do  work,  an  animal  must  be  fed ;  and,  even  at  a  still 
earlier  period,  Count  Rumford  remarked  that  a  ton  of  hay 
will  be  administei'ed  more  economically  by  feeding  a  horse 
with  it,  and  then  getting  work  out  of  the  horse,  than  by 
burning  it  as  fuel  in  an  engine. 

225.  In  this  chapter,  the  same  line  of  thought  has 
been  carried  out  a  little  further.  We  have  seen  that  life 
is  associated  with  delicately-constructed  machines,  so 
that  whenever  a  transmutation  of  energy  is  brought 
about  by  a  living  being,  could  we  trace  the  event  back, 
we  should  find  that  the  physical  antecedent  was  probably 
a  much  less  transmutation,  while  again  the  antecedent  of 
this  would  probably  be  found  still  less,  and  so  on,  as  far 
as  we  could  trace  it. 

226.  But  with  all  this,  we  do  not  pretend  to  have  dis- 
covered the  true  nature  of  life  itself,  or  even  the  true 
nature  of  its  relation  to  the  material  universe. 

What  we  have  ventured  is  the  assertion  that,  as  far  as 
we  can  judge,  life  is  always  associated  with  machinery  of 
a  certain  kind,  in  virtue  of  which  an  extremely  delicate 
directive  touch  is  ultimately  magnified  into  a  very  con- 
siderable transmutation  of  energy.  Indeed,  we  can 
hardly  imagine  the  freedom  of  motion  implied  in  life 
to  exist  apart  from  machinery  possessed  of  very  gi-eat 
delicacy  of  construction. 

In  fine,  we  have  not  succeeded  in  solving  the  problem 
as  to  the  true  nature  of  life,  but  have  only  driven 
the  difficulty  into  a  borderland  of  thick   darkness,  into 


164?  TUE   CONSERVATION   OF   ENEEQY. 

which  the  light  of  knowledge  has  not  yet  been  able  to 
penetrate. 

Organized  Tissues  are  subject  to  Decay. 

227.  We  have  thus  learned  two  things,  for,  in  the 
first  place,  we  have  learned  that  life  is  associated  with 
delicacy  of  construction,  and  in  the  next  (Art.  220),  that 
delicacy  of  construction  implies  an  unstable  arrangement 
of  natural  forces.  We  have  now  to  remark  that  the 
particular  force  which  is  thus  used  by  living  beings  is 
chemical  affinity.  Our  bodies  are,  in  truth,  examples  of 
an  unstable  arrangement  of  chemical  forces,  and  the 
materials  which  composed  them,  if  not  liable  to  sudden 
explosion,  like  fulminating  powder,  are  yet  pre-eminently 
the  subjects  of  decay. 

228.  Now,  this  is  more  than  a  mere  general  statement ; 
it  is  a  truth  that  admits  of  degrees,  and  in  virtue  of 
which  those  parts  of  our  bodies  which  have,  during  life, 
the  noblest  and  most  delicate  ofiice  to  perform,  are  the 
very  first  to  perish  when  life  is  extinct. 

"  Oh  !  o'er  the  eye  death  most  exerts  his  might, 
And  hurls  the  spirit  from  her  throne  of  light ; 
Sinks  those  blue  orbs  in  their  long  last  eclipse, 
But  spares  as  yet  the  charm  around  the  lips." 

So  speaks  the  poet,  and  we  have  here  an  aspect  of 
things  in  wdiich  the  lament  of  the  poet  becomes  the  true 
interpretation  of  nature. 


THE  POSITION    OF   LIFE.  1G5 

Difference  between  Animals  and  Inanimate 
Machines. 

229.  We  are  now  able  to  recognize  the  difference  be- 
tween the  relations  -to  energy  of  a  living  being,  such  as 
man,  and  a  machine,  such  as  a  steam-engina 

There  are  many  points  in  common  between  the  two. 
Both  require  to  be  fed,  and  in  both  there  is  the  transmu- 
tation of  the  energy  of  chemical  separation  implied  in 
fuel  and  food  into  that  of  heat  and  visible  motion. 

But  while  the  one — the  engine — requires  for  its  main- 
tenance only  carbon,  or  some  other  variety  of  chemical 
separation,  the  other — the  living  being — demands  to  be 
supplied  with  organized  tissue.  In  fact,  that  delicacy  of 
construction  which  is  so  essential  to  our  well-being,  is 
not  something  which  we  can  elaborate  internally  in  our 
own  frames — all  that  we  can  do  is  to  appropriate  and 
assimilate  that  which  comes  to  us  from  without;  it  is 
already  present  in  the  food  which  we  eat. 

Ultimate  Dependence  of  Life  upon  the  Sun. 

230.  We  have  already  (Art.  203J  been  led  to  recognize 
the  sun  as  the  ultimate  material  source  of  all  the  energy 
which  we  possess,  and  we  must  now  regard  him  as  the 
source  likewise  of  all  our  delicacy  of  construction.  It 
requires  the  energy  of  his  high  temperature  rays  so  to 
wield  and  manipulate  the  powerful  forces  of  chemical 
affinity;  so  to  balance  these  various  forces  against  each 


163  THE   CONSERVATION    OF   ENERGY. 

other,  as  to  produce  in  the  vegetable  something  which 
will  afford  our  frames,  not  only  energy,  but  also  delicacy 
of  construction. 

Low  temperature  heat  would  be  utterly  unable  to 
accomplish  this  ;  it  consists  of  ethereal  vibrations  which 
are  not  sufficiently  rapid,  and  of  waves  that  are  not  suffi- 
ciently short,  for  the  purpose  of  shaking  asunder  the 
constituents  of  compound  molecules. 

231.  It  thus  appears  that  animals  are,  in  more  ways 
than  one,  pensioners  upon  the  sun's  bounty;  and  those 
instances,  which  at  first  sight  appear  to  be  exceptions, 
will,  if  studied  sufficiently,  only  serve  to  confirm  the  rule. 

Thus  the  recent  researches  of  Dr.  Carpenter  and  Pro- 
fessor Wy  ville  Thomson  have  disclosed  to  us  the  existence 
of  miimte  living  beings  in  the  deepest  parts  of  the  ocean, 
into  which  we  may  be  almost  sure  no  solar  ray  can 
penetrate.  How,  then,  do  these  minute  creatures  obtain 
that  energy  and  delicacy  of  construction  without  which 
they  cannot  live  ?  in  other  words,  how  are  they  fed  ? 

Now,  the  same  naturalists  who  discovered  the  exist- 
enC3  of  these  creatures,  have  recently  furnished  us  with 
a  very  probable  explanation  of  the  mystery.  They  think 
it  highly  probable  that  the  whole  ocean  contains  in 
it  organic  matter  to  a  very  small  but  yet  perceptible 
extent,  forming,  as  they  express  it,  a  soit  of  diluted  soup, 
whicli  thus  becomes  the  food  of  these  minute  creatures. 

232.  In  conclusion,  we  are  dependent  upon  the  sun  and 
centre  of  our  system,  not  only  for  the  mere  energy  of  our 


THE  POSITION   OF   LIFK  1G7 

frames,  but  also  for  our  delicacy  of  construction — the 
future  of  our  race  depends  upon  the  sun's  future.  But 
we  have  seen  that  the  sun  must  have  had  a  beofinnincc, 
and  that  he  will  have  an  end. 

We  are  thus  induced  to  generalize  still  further,  and 
regard,  not  only  our  own  system,  but  the  whole  material 
universe  when  viewed  with  respect  to  serviceable  energy, 
as  essentially  evanescent,  and  as  embracing  a  succession 
of  physical  events  which  cannot  go  on  for  ever  as  they 
are. 

But  here  at  length  we  come  to  matters  beyond  our 
grasp ;  for  physical  science  cannot  inform  us  what  must 
have  been  before  the  beginning,  nor  yet  can  ifc  tell  ua 
what  will  take  place  aftf^r  the  end. 


APPEI^DIX 


CORRELATION  OF  VITAL  WITH  CHEMICAL  AND 
PHYSICAL  FORCES. 

By  JOSEPH  LE   CONTE, 

PBOFESSOR   OF    GEOLOGY    AND    NATURAL    HISTORY    IN    THE 
UNIVERSITY    OF  CALIFORNIA. 


CORRELATION  OF  YITAL  WITH  CHEMICAL 
AND  PHYSICAL  FORCES. 


YiTAL  force  ;  whence  is  it  derived  ?  "What  is  its  re- 
lation to  tlie  other  forces  of  J^ature  ?  The  answer  of 
modern  science  to  these  questions  is  :  It  is  derived  from 
the  lower  forces  of  Nature  ;  it  is  related  to  other  forces 
much  as  these  are  related  to  each  other — it  is  correlated 
with  chemical  and  physical  forces. 

At  one  time  matter  was  supposed  to  be  destructible. 
By  combustion  or  by  evaporation  matter  seemed  to  be 
consumed — to  pass  out  of  existence  ;  but  now  we  know 
it  only  changes  its  form  from  the  solid  or  liquid  to  the 
gaseous  condition — ^from  the  visible  to  the  invisible — 
and  that,  amid  all  these  changes,  the  same  quantity  of 
matter  remains.  Creation  or  destruction  of  matter,  in- 
crease or  diminution  of  matter,  lies  beyond  the  domain 
of  Science ;  her  domain  is  confined  entu-ely  to  the 
changes  of  matter.  K'ow,  it  is  the  doctrine  of  modern 
science  that  the  same  is  true  of  force.     Force  seems  of- 


172        THE  CONSEEVATION  OF  ENERGY. 

ten  to  be  annihilated.  Two  cannon-balls  of  equal -size 
and  velocity  meet  each  other  and  fall  motionless.  The 
immense  energy  of  these  moving  bodies  seems  to  pass 
out  of  existence.  But  not  so  ;  it  is  changed  into  heat, 
and  the  exact  amount  of  heat  may  be  calculated ;  more- 
over, an  equal  amount  of  heat  may  be  changed  back 
again  into  an  equal  amount  of  momentum.  Here,  there- 
fore, force  is  not  lost,  but  is  changed  from  a  visible  to 
an  invisible  form.  Motion  is  changed  from  bodily  mo- 
tion into  molecular  motion.  Thus  heat,  light,  electrici- 
ty, magnetism,  chemical  affinity,  and  mechanical  force, 
are  transmutable  into  each  other,  back  and  forth  ;  but, 
amid  all  these  changes,  the  amount  of  force  remains  un- 
changed. Force  is  incapable  of  destruction,  except  by 
the  same  power  which  created  it.  The  domain  of  Sci- 
ence lies  within  the  limits  of  these  changes — creation  and 
annihilation  lie  outside  of  her  domain. 

The  mutual  convertibility  of  forces  into  each  other 
is  called  correlation  of  forces ;  the  persistence  of  the 
same  amount,  amid  all  these  protean  forms,  is  called 
conservation  of  force. ^ 

*  In  recent  works  the  word  energy  is  used  to  designate  active  or  work- 
ing force  as  distinguished  from  passive  or  non-working  force.  It  is  in  this 
working  condition  only  that  force  is  conserved,  and  therefore  conservation 
of  energy  is  the  proper  expression.  Nevertheless,  since  the  distinction 
between  force  and  energy  is  imperfectly  or  not  at  all  defined  in  the  higher 
forms  of  force,  and  especially  in  the  domain  of  life,  I  have  preferred  in 
this  article  to  use  the  word  force  in  the  general  sense  usual  until  recently. 


PHYSICAL,   CHEMICAL,   AND   VITAL  FORCES.  173 

The  correlation  of  pliysical  forces  with  each  other 
and  with  chemical  force  is  now  universally  acknowl- 
edged and  somewhat  clearly  conceived.  The  correla- 
tion  of  vital  force  with  these  is  not  universally  acknowl- 
edged, and,  where  acknowledged,  is  only  imperfectly 
conceived.  In  1859  I  published  a  paper*  in  which  I 
attempted  to  put  the  idea  of  correlation  of  vital  force 
with  chemical  and  physical  forces  in  a  more  definite 
and  scientific  form.  The  views  expressed  in  that  paper 
have  been  generally  adopted  by  physiologists.  Since 
the  publication  of  the  paper  referred  to,  the  subject  has 
lain  in  my  mind,  and  grown  at  least  somewhat.  I  pro- 
pose, therefore,  now  to  reembody  my  views  in  a  more 
popular  form,  with  such  additions  as  have  occurred  to 
me  since. 

There  are  four  planes  of  material  existence,  which 
may  be  represented  as  raised  one  above  another.  These 
are  :  1.  The  plane  of  elementary  existence ;  2.  The  plane 
of  chemical  compounds,  or  mineral  kingdom ;  3.  The 
plane  of  vegetable  existence  ;  and,  4.  The  plane  of  ani- 
mal existence.  Their  relations  to  each  other  are  truly 
expressed  by  writing  them  one  above  the  other,  thus  : 

I  may  sometimes  use  the  word  energy  instead.  If  any  one  should  charge 
me  with  want  of  precision  in  language,  my  answer  is  :  Our  language  cannot 
be  more  precise  until  our  ideas  in  this  department  are  far  clearer  than  now. 
*  American  Journal  of  Science,  November,  1859.  Philadelphia  Maga- 
zine, vol.  xis.,  p.  133. 


£74  THE  CONSERVATION  OF  ENERGY. 

4.  Animal  kingdom. 
3.   Vegetable  Kingdom. 
2.  Mineral  Kingdom. 
1.  Elements. 

!N^ow,  it  is  a  remarkable  fact  that  there  is  a  special 
force,  whose  function  it  is  to  raise  matter  from  each 
plane  to  the  plane  above,  and  to  execute  movements  on 
the  latter.  Thus,  it  is  the  function  of  chemical  affinity 
alone  to  raise  matter  from  ISTo.  1  to  No.  2,  as  well  as  to 
execute  all  the  movements,  back  and  forth,  by  action 
and  reaction  ;  in  a  word,  to  produce  all  the  phenomena 
on  No.  2  which  together  constitute  the  science  of  chem- 
istry. It  is  the  prerogative  of  vegetable  life-force  alone 
to  lift  matter  from  No.  2  to  No.  3,  as  well  as  to  execute 
all  the  movements  on  that  plane,  which  together  consti- 
tute tlie  science  of  vegetable  physiology.  It  is  the  pre- 
rogative of  animal  life-force  alone  to  lift  matter  from 
No.  3  to  No.  4,  and  to  preside  over  the  movements  on 
this  plane,  which  together  constitute  the  science  of  ani- 
mal physiology.  But  there  is  no  force  in  Nature  capa- 
ble of  raising  matter  at  once  from  No.  1  to  No.  3,  or 
from  No.  2  to  No.  4,  without  stopping  and  receiving  an 
accession  of  force,  of  a  different  kind,  on  the  intermedi- 
ate plane.  Plants  cannot  feed  upon  elements,  but  only 
on  chemical  compounds ;  animals  cannot  feed  on  min- 
erals, but  only  on  vegetables.    We  shall  sec  in  the  sequel 


PHYSICAL,   CHEMICAL,  AND  VITAL  FORCES.  I75 

that  this  is  the  necessary  result  of  the  princij^le  of  con- 
Bervation  of  force  in  vital  phenomena. 

It  is  well  known  that  atoms,  in  a  nascent  state — i.  e., 
at  the  moment  of  their  separation  from  previous  com- 
bination— are  endowed  with  peculiar  and  powerful  af- 
Unity.  Oxygen  and  nitrogen,  nitrogen  and  hydrogen, 
hydrogen  and  carbon,  which  show  no  affinity  for  each 
other  under  ordinary  circumstances,  readily  unite  when 
one  or  both  are  in  a  nascent  condition.  The  reason  seems 
to  be  that,  when  the  elements  of  a  compound  are  torn 
asunder,  the  chemical  affinity  which  previously  bound 
them  together  is  set  free,  ready  and  eager  to  unite  the 
nascent  elements  with  wdiatever  they  come  in  contact 
with.  This  state  of  exalted  chemical  energy  is  retained 
but  a  little  while,  because  it  is  liable  to  be  changed  into 
some  other  form  of  force,  probably  heat,  and  is  there- 
fore no  longer  chemical  energy.  .To  illustrate  by  the 
planes  :  matter  falling  down  from  No.  2  to  Xo.  1  gener- 
ates force  by  which  matter  is  lifted  from  'No.  1  to  No. 
2.  Decomposition  generates  the  force  by  which  combi- 
nation is  effected.  This  princij^le  underlies  every  thing 
I  shall  further  say. 

There  are,  therefore,  two  ideas  or  principles  under- 
lying this  paper :  1.  The  correlation  of  vital  with  phys- 
ical and  chemical  forces  ;  2.  That  in  all  cases  viial  force 
is  fi'oduced  hy  decomjjositioii — is  transformed  nascent 
affinity.     Neither  of  these  is  new.     Grove,  many  years 


176  THE   CONSEEVATION   OF  ENERGY. 

ago,  brouglit  out,  in  a  vague  manner,  tlie  idea  that 
vital  force  was  correlated  with  cliemical  and  physical 
forces.*  In  IS-IS  Dr.  Frelce,  M.  K.  I.  A.,  of  Dublin, 
first  advanced  the  idea  that  vital  force  of  animal  life 
was  generated  by  decomposition.  In  1851  the  same 
idea  was  brought  out  again  by  Dr.  Watters,  of  St.  Louis. 
These  papers  were  unknown  to  me  when  I  wrote  my 
article.  They  have  been  sent  to  me  in  the  last  few 
years  by  their  respective  authors.  ITeither  of  these  au- 
thors, however,  extends  this  principle  to  vegetation,  the 
most  fundamental  and  most  important  phenomenon  of 
life.  In  1857  the  same  idea  was  again  brought  out  by 
Prof.  Henry,  of  the  Smithsonian  Institution,  and  by 
him  extended  to  vegetation.  I  do  not,  therefore,  now 
claim  to  have  first  advanced  this  idea,  but  I  do  claim  to 
have  in  some  measure  rescued  it  fi*om  vagueness,  and 
given  it  a  clearer  and  more  scientific  form. 

I  wish  now  to  apply  these  principles  in  the  explana- 
tion of  the  most  important  phenomena  of  vegetable  and 
animal  life  : 

1.  Yegetation. — The  most  important  phenomenon  in 
the  life-history  of  a  plant — in  fact,  the  starting-point  of 
all  life,  both  vegetable  and  animal — is  the  formation  of 
organic  matter  in  the  leaves.  The  necessary  conditions 
for  this  wonderful  change  of  mineral  into  organic  mat- 

*  la  1845  Dr.  J.  R.  Mayer  published  a  paper  on  "  Organic  Motion  and 
Nutrition."     I  have  not  seen  it. 


PHYSICAL,   CHEMICAL,  AND  VITAL  FORCES.  177 

ter  seem  to  be,  sunlight,  cliloropliyl,  aud  living  proto- 
plasm, or  bioplasm.  This  is  tlie  phenomenon  I  wish 
now  to  discuss. 

The  j)lastic  matters  of  which  vegetable  structure  is 
built  are  of  two  kinds — amyloids  and  albuminoids.  The 
amyloids,  or  starch  and  sugar  groups,  consist  of  C,  H, 
and  O  ;  the  albuminoids  of  C,  H,  O,  ^N",  and  a  little  S 
and  P.  The  quantity  of  sulphur  and  phosphorus  is  very 
small,  and  we  will  neglect  them  in  this  discussion.  The 
food  out  of  which  these  substances  are  elaborated  are, 
GO2,  H2O,  and  H3N — carbonic  acid,  water,  and  ammonia. 
Now,  by  the  agency  of  sunlight  in  the  presence  of 
chlorophyl  and  bioplasm,  these  chemical  compounds 
(CO2,  II2O,  HolST)  are  torn  asunder,  or  shaken  asunder, 
or  decomposed  ;  the  excess  of  O,  or  of  O  and  H,  is  re- 
jected, and  the  remaining  elements  in  a  nascent  condi- 
tion, combine  to  form  organic  matter.  To  form  the 
amyloids — starch,  dextrine,  sugar,  cellulose — only  CO2 
and  H2O  are  decomposed,  and  excess  of  O  rejected.  To 
form  albuminoids,  or  protoplasm,  CO2,  H2O,  and  Hsl^, 
are  decomposed,  and  excess  of  O  and  H  rejected. 

It  would  seem  in  this  case,  therefore,  that  physical 
force  (light)  is  changed  into  nascent  chemical  force,  and 
this  nascent  chemical  force,  under  the  peculiar  condi- 
tions present,  forms  oi-ganic  matter,  and  reappears  as 
vital  force.  Light  falling  on  living  green  leaves  is  de- 
stroyed or  consumed  in  doing  the  work  of  decomposi- 


[78  THE  CONSEEVATION   OF  ENEF.Gr. 

tion  ;  disappears  as  liglit,  to  reappear  as  nascent  clieuii- 
cal  energy  ;  and  this  in  its  turn  disappears  in  forming 
organic  matter,  to  reappear  as  the  vital  force  of  the  or- 
ganic matter  thus  formed.  The  light  which  disappears 
is  proportioned  to  the  O,  or  the  O  and  H  rejected ;  is 
proportioned  also  to  the  quantity  of  organic  matter 
formed,  and  also  to  the  amount  of  vital  force  resulting. 
To  illustrate :  In  the  case  of  amyloids,  oxygen-excess 
falling  or  running  down  from  plane  JSTo.  2  to  plane  No. 

1  generates  force  to  raise  C,  II,  and  O,  from  plane  No. 

2  to  plane  No.  3.  In  the  case  of  albuminoids,  oxygen- 
excess  and  hydrogen-excess  running  down  from  No.  2 
to  No.  1  generate  force  to  raise  C,  H,  O,  and  N,  from 
No.  2  to  No.  3.  To  illustrate  again  :  As  sun-heat  fall- 
ing upon  water  disappears  as  heat,  to  reappear  as  me- 
chanical power,  raising  the  water  into  the  clouds,  so 
sunlight  falling  upon  green  leaves  disappears  as  light,  to 
reappear  as  vital  force  lifting  matter  from  the  mineral 
into  the  organic  kingdom. 

2.  Gekmestation. — Growing  plants,  it  is  seen,  take 
their  life-force  from  the  sun  ;  but  seeds  germinate  and 
commence  to  grow  in  the  dark.  Evidently  there  must 
be  some  other  source  from  which  they  draw  their  sup- 
ply of  force.  They  cannot  draw  force  from  the  sun. 
This  fact  is  intimately  connected  with  another  fact,  viz., 
that  they  do  not  draw  their  food  from  the  mineral  king- 
dom.    The  seed  in  germination  feeds  entirely  upon  a 


PHYSICAL,  CHEMICAL,   AND  VITAL  FOKCES.  179 

Bupply  of  organic  matter  laid  up  for  it  by  tlie  mother- 
plant.  It  is  tlie  decomposition  of  this  organic  matter 
wliicli  supplies  tlie  force  of  germination.  Chemical 
compounds  are  comparatively  stable — it  requires  sun- 
light to  tear  them  asunder  ;  but  organic  matter  is  more 
easily  decomposed — it  is  almost  spontaneously  decom- 
posed. It  may  be  that  heat  (a  necessary  condition  of 
germination)  is  the  force  which  determines  the  decom- 
position. However  this  may  be,  it  is  certain  that  a  por- 
tion of  the  organic  matter  laid  up  in  the  seed  is  decora- 
posed,  burned  up,  to  form  CO2  and  H2O,  and  that  this 
combustion  furnishes  the  force  by  which  the  mason- 
work  of  tissue-making  is  accomplished.  In  other  words, 
of  the  food  laid  up  in  the  form  of  starch,  dextrine,  pro- 
toplasm, a  portion  is  decomposed  to  furnish  the  force  by 
which  the  remainder  is  organized.  Hence  the  seed  al- 
ways loses  weight  in  germination  ;  it  cannot  develop 
unless  it  is  in  part  consumed  ;  "  it  is  not  quickened  ex- 
cept it  die."  This  self-consumption  continues  until  the 
leaves  and  roots  are  formed ;  then  it  begins  to  draw 
force  from  the  sun,  and  food  from  the  mineral  kingdom.. 
To  illustrate :  In  germination,  matter  running  down 
from  plane  !N'o.  3  to  plane  No.  2  generates  force  by 
which  other  similar  matter  is  moved  about  and  raised 
to  a  somewhat  higher  position  on  plane  ISo.  3.  As 
water  raised  by  the  sun  may  be  stored  in  reservoirs,  and 
ill  running  down  from  these  may  do  work,  so  matter 


180  THE   CONSERVATION  OF  ENEEGY. 

raised  by  sun-force  into  the  organic  kingdom  by  one 
generation  is  stored  as  force  to  do  tbe  work  of  germina- 
tion of  the  next  generation.  Again,  as^  in  water  run- 
ning through  an  hydraulic  ram,  a  portion  runs  to  waste, 
in  order  to  generate  force  to  lift  the  remainder  to  a 
higher  level,  so,  of  organic  matter  stored  in  the  seed,  a 
portion  runs  to  waste  to  create  force  to  organize  the  re- 
mainder. 

Thus,  then,  it  will  be  seen  that  three  things,  viz.,  the 
absence  of  sunlight,  the  use  of  organic  food,  and  the 
loss  of  weight,  are  indissolubly  connected  in  germina- 
tion, and  all  explained  by  the  principle  of  conservation 
of  force. 

3.  Staeting  of  Buds. — Deciduous  trees  are  entirely 
destitute  of  leaves  during  the  winter.  The  buds  must 
start  to  grow  in  the  spring  without  leaves,  and  there- 
fore without  drawing  force  from  the  sun.  Hence,  also, 
food  in  the  organic  form  must  be,  and  is,  laid  up  from 
the  previous  year  in  the  body  of  the  tree.  A  portion 
of  this  is  consumed  with  the  formation  of  COg  and 
H2O,  in  order  to  create  force  for  the  development  of  the 
buds.  So  soon  as  by  this  means  the  leaves  are  formed, 
the  plant  begins  to  draw  force  from  the  sun,  and  food 
from  the  mineral  kingdom. 

4.  Pale  Pla^jts. — Fungi  and  etiolated  plants  have 
no  chlorophyl,  therefore  cannot  draw  their  force  from 
the   sun,   nor  make   organic  matters   from   inorganic. 


PHYSICAL,  CHEMICAL,   AND  VITAL  FORCES.  ISl 

Hence  tliese  also  must  feed  on  organic  matter  ;  not,  in- 
deed, on  starch,  dextrine,  and  protoplasm,  but  on  de- 
caying organic  matter.  In  these  plants  the  organic 
matter  is  taken  up  in  some  form  intermediate  between 
the  planes  IsTo.  3  and  Ko.  2.  The  matter  thus  taken  up 
is,  a  portion  of  it,  consumed  with  the  foi-mation  of  CO2 
and  H2OJ  in  order  to  create  force  necessary  to  organize 
the  remainder.  To  illustrate  :  Matter  falling  from 
some  intermediate  point  between  Xo.  2  and  jSTo.  3  to 
ISTo.  2,  produces  force  sufficient  to  raise  matter  from  the 
same  intermediate  point  to  ]^o.  3  ;  a  portion  runs  to 
waste  downward,  and  creates  force  to  push  the  remain- 
der upward. 

5.  Gkowth  of  Geeen  Plajcts  at  Xight. — It  is  Avell 
known  that  almost  all  plants  grow  at  night  as  well  as  in 
the  day.  It  is  also  known  that  plants  at  night  exhale 
CO2.  These  two  facts  have  not,  however,  as  far  as  I 
know,  been  connected  with  one  another,  and  with  the 
principle  of  conservation  of  force.  It  is  usually  sup- 
posed that  in  the  night  the  decomposition  of  CO2  and 
exhalation  of  oxygen  are  checked  by  withdrawal  of  sun 
light,  and  some  of  the  COo  in  the  ascending  sap  is  ex- 
haled by  a  physical  law.  But  this  does  not  account  for 
the  growth.  It  is  evident  that,  in  the  absence  of  sun- 
light, the  force  required  for  the  work  of  tissue-buiiding 
can  be  derived  only  fi-om  the  decomposition  and  com- 
bustion of  oro-anic  matter,     Tliere  are  two  views  as  to 


182        THE  CONSERVATION  OF  ENEEGY. 

the  source  of  this  organic  matter,  either  or  both  of  which 
may  be  correct :  First.  There  seems  to  be  no  doubt  that 
most  plants,  especially  those  grown  in  soils  rich  in  /m- 
muSj  take  up  a  portion  of  their  food  in  the  form  of  semi- 
organic  matter,  or  soluble  huonus.  The  combustion  of  a 
portion  of  this  in  every  part  of  the  plant,  by  means  of 
oxygen  also  absorbed  by  the  roots,  and  the  formation 
of  CO2,  undoubtedly  creates  a  supply  of  force  night  and 
day,  indGr]3endently  of  sunlight.  The  force  thus  pro- 
duced by  the  combustion  of  a  portion  might  be  used  to 
raise  the  remainder  into  starch,  dextrine,  etc.,  or  might 
be  used  in  tissue-building.  During  the  day,  the  COj 
thus  produced  would  be  again  decomposed  in  the  leaves 
by  sunlight,  and  thus  create  an  additional  supply  of 
force.     During  the  night,  the  CO^  would  be  exhaled.* 

Again  :  It  is  possible  that  more  organic  matter  is 
made  by  sunlight  during  the  day  than  is  used  up  in  tis- 
sue-building. Some  of  this  excess  is  again  consumed, 
and  forms  CO2  and  IT2O,  in  order  to  continue  the  tissue- 
building  process  during  the  night.  Thus  the  plant  dur 
ing  the  day  stores  up  sun-force  sufficient  to  do  its  work 
during  the  night.  It  has  been  suggested  by  Dr.  J.  C. 
Draper,f  though  not  proved,  or  even  rendered  probable, 

*  For  more  full  account,  see  my  paper,  American  Journal  of  Science, 
November,  1859,  sixth  and  seventh  heads. 

\  American  Journal  of  Science, 'November,  18Y2.  The  experiments  of 
Dr.  Draper  are  incon'-lusive,  because  they  are  made  on  seedlings,  which. 


PHYSICAL,  CHEMICAL,   AND   VITAL  FORCES.         lg3 

that  tlie  force  of  tissue-building  {force  plastique)  is  al- 
ways derived  from  decomposition,  or  combustion  of  or- 
ganic matter.  In  that  case,  tlie  force  of  organic-matter 
formation  is  derived  from  the  sun,  wliile  tlie  force  of 
tissue  -  building  (wliicli  is  relatively  small)  is  derived 
from  tlie  combustion  of  organic  matter  tlius  previously 
formed. 

6.  Feementatiok. — The  plastic  matters  out  of  which 
vegetable  tissue  is  built,  and  which  are  formed  by  sun- 
light in  the  leaves,  are  of  two  kinds,  viz.,  amyloids 
(dextrine,  sugar,  starch,  cellulose),  and  albuminoids,  or 
protoplasm.  Kow,  the  amyloids  are  comparatively  sta- 
ble, and  do  not  spontaneously  decompose ;  but  the  albu- 
minoids not  only  decompose  spontaneously  themselves, 
but  drag  down  the  amyloids  with  which  they  are  asso- 
ciated into  concurrent  decomposition — not  only  change 
themsel'\ies,  but  propagate  a  change  into  amyloids.  Al- 
buminoids, in  various  stages  and  kinds  of  decomposi- 
tion, are  called  ferments.  The  propagated  change  in 
amyloids  is  called  fermentation.  By  various  kinds  of 
ferments,  amyloids  arc  thus  dragged  down  step  by  step 
to  the  mineral  kingdom,  viz.,  to  CO2  and  H2O.  The 
accompanying  table  exhibits  the  various  stages  of  the 
descent  of  starch,  and  the  ferments  by  which  they  are 
effected : 

until  their  supply  of  organic  food  is  exhausted,  are  independent  of  sun 
asht. 


184  THE  CONSERVATION  OF  ENERGY. 

1.  Starch ^ 

2.  Dextrine v  Diastase. 

3.  Sugar ) 

4.  Alcohol  and  COj Yeast. 

5.  Acetic  acid Mother  of  vinegar. 

6.  COj  and  H^O Mould. 

Bj  appropriate  means,  tlie  process  of  descent  mar 
be  stopped  on  any  one  of  tliese  planes.  JBj  far  too 
miicli  is,  unfortunately,  stopped  on  the  fourth  plane. 
The  manufacturer  and  chemist  may  determine  the 
downward  change  through  all  the  planes,  and  the  chem- 
ist has  recently  succeeded  in  ascending  again  to  l^o.  4  ; 
but  the  plant  ascends  and  descends  the  scale  at  pleasure 
(avoiding,  however,  the  fourth  and  fifth),  and  even  passes 
at  one  step  from  the  lowest  to  the  highest. 

Now,  it  will  be  seen  by  the  table  that,  connected 
with  each  of  these  descensive  changes,  there  is  a  peculiar 
ferment  associated.  Diastase  determines  the  change 
from  starch  to  dextrine  and  sugar  —  saccharification  ; 
yeast,  the  change  from  sugar  to  alcohol — fermentation  ; 
mother  of  vinegar,  the  change  from  alcohol  to  acetic 
acid — acetification  ;  and  a  peculiar  mould,  the  change 
from  acetic  acid  to  CO^  and  water.  But  what  is  far 
more  wonderful  and  significant  is,  that,  associated  with 
each  of  these  ferments,  except  diastase,  and  therefore 
with  each  of  these  descensive  changes,  except  the  change 
from  starch  to  sugar,  or  saccharification,  there  is  a  pecul- 


PHYSICAL,   CHEMICAL,  AND  VITAL  i'OKCES.         Igj 

lar  form  of  life.  Associated  with  alcoholic  fermenta- 
tion, there  is  the  yeast-plant ;  with  acetification,  the  vin- 
egar-plant ;  and  with  the  decomposition  of  vinegar,  a 
peculiar  kind  of  mould.  We  will  take  the  one  which  is 
best  understood,  viz.,  yeast-plant  (saccharomyce),  and 
its  relation  to  alcoholic  fermentation. 

It  is  well  known  that,  in  connection  with  alcoholic 
fermentation,  there  is  a  peculiar  unicelled  plant  which 
grows  and  multiplies.  Fermentation  never  takes  place 
without  the  presence  of  this  plant ;  this  plant  never 
grows  without  producing  fermentation,  and  the  rapidity 
of  the  fermentation  is  in  exact  proportion  to  the  rapid- 
ity of  the  growth  of  the  plant.  But,  as  far  as  I  know, 
the  fact  has  not  been  distinctly  brought  out  that  the  de- 
composition of  the  sugar  into  alcohol  and  carbonic  acid 
furnishes  the  force  by  which  the  plant  grows  and  multi- 
plies. If  the  growing  cells  of  the  yeast-plant  be  ob- 
served under  the  microscope,  it  will  be  seen  that  the 
carbonic-acid  bubbles  form,  and  therefore  probably  the 
decomposition  of  sugar  takes  place  only  in  contact  with 
the  surface  of  the  yeast-cells.  The  yeast-plant  not  only 
assimilates  matter,  but  also  force.  It  decomposes  the 
sugar  in  order  that  it  may  assimilate  the  chemical  force 
set  free. 

"We  have  already  said  that  the  change  from  starch  to 
sugar,  determined  by  diastase  (saccharification),  is  the 
only  one  in   connection  with  which  there  is  no  life. 


[8G  TliE  CONSERVATION   OF  ENEEGY. 

Now,  it  is  a  most  significant  fact,  in  tliis  connection,  tLat 
this  is  also  the  only  change  which  is  not,  in  a  proper  sense, 
descensive,  or,  at  least,  where  there  is  no  decomposition. 

We  now  pass  from  the  phenomena  of  vegetable  to 
the  phenomena  of  animal  life. 

7.  Development  of  the  Egg  in  Incubation. — The 
development  of  the  egg  in  incubation  is  very  similar  to 
the  germination  of  a  seed.  An  egg  consists  of  albumi- 
nous and  fatty  matters,  so  inclosed  that,  while  oxygen  of 
the  air  is  admitted,  nutrient  matters  are  excluded.  Dur- 
ing incubation  the  egg  changes  into  an  embryo ;  it 
passes  from  an  almost  unorganized  to  a  highly-organ- 
ized condition,  from  a  lower  to  a  higher  condition. 
There  is  work  done :  there  must  be  expenditure  of 
force ;  but,  as  we  have  already  seen,  vital  force  is  al- 
ways derived  from  decomposition.  But,  as  the  matters 
to  be  decomposed  are  not  taken  db  extra,  the  egg  must 
consume  itself ;  that  it  does  so,  is  proved  by  the  fact 
that  in  incubation  the  egg  absorbs  oxygen,  eliminates 
CO2  and  probably  H2OJ  aiid  loses  weight.  As  in  the 
seed,  a  portion  of  the  matters  contained  in  the  egg  is 
consumed  in  order  to  create  force  to  organize  the  re- 
mainder. Matter  runs  down  from  plane  No.  4  to  plane 
No.  2,  and  generates  force  to  do  the  work  of  organiza- 
tion on  plane  No.  4.  The  amount  of  CO2  and  HgO 
formed,  and  therefore  the  loss  of  weight,  is  a  measure 
of  the  amount  of  plastic  work  done. 


PHYSICAL,  CHEMICAL,  AKD  VITAL  FOECES.  187 

8.  Development  wrrnm  the  Chkysalis  Shell. — It 
is  well  known  that  many  insects  emerge  from  the  egg 
not  in  their  final  form,  but  in  a  wormlike  form,  called  a 
larva.  After  this  they  pass  into  a  second  passive  state, 
in  which  they  are  again  covered  with  a  kind  of  shell — a 
sort  of  second  egg-state,  called  the  chrysalis.  From  this 
they  again  emerge  as  the  perfect  insect.  The  butterfly 
is  the  most  familiar,  as  well  as  the  best,  illustration  of 
these  changes.  The  larva  or  caterpillar  eats  with  en  or. 
mous  voracity,  and  grows  very  rapidly.  When  its 
growth  is  complete,  it  covers  itself  with  a  shell,  and  re- 
mains perfectly  passive  and  almost  immovable  for  many 
days  or  weeks.  During  this  period  of  quiescence  of  ani- 
mal functions  there  are,  however,  the  most  important 
changes  going  on  within.  The  wings  and  legs  are 
formed,  the  muscles  are  aggregated  in  bundles  for  mov- 
ing these  appendages,  the  nervous  system  is  more  high- 
ly developed,  the  mouth-organs  and  alimentary  canal 
are  greatly  changed  and  more  highly  organized,  the 
simple  eyes  are  changed  into  compound  eyes.  ISTow,  all 
this  requires  expenditure  of  force,  and  therefore  decom- 
position of  matter ;  but  no  food  is  taken,  therefore  the 
chrysalis  must  consume  its  own  substance,  and  therefore 
lose  weight.  It  does  so  ;  the  weight  of  the  emerging 
butterfly  is  in  many  cases  not  one-tenth  that  of  the 
caterpillar.  Force  is  stored  up  in  the  form  of  organic 
matter  only  to  be  consumed  in  doing  plastic  work. 


[88         THE  CONSERVATION. OF  ENERGY. 

9.  Mature  Animals. — Whence  do  animals  derive 
tlieir  vital  force  ?  I  answer,  from  tlie  decomposition  of 
their  food  and  the  decomposition  of  their  tissues. 

Plants,  as  we  have  seen,  derive  their  vital  force  from 
the  decomposition  of  their  mineral  food.  But  the  chem- 
ical compounds  on  which  plants  feed  are  very  stable. 
Their  decomposition  requires  a  peculiar  and  complex 
contrivance  for  the  reception  and  utilization  of  sunlight. 
These  conditions  are  wanting  in  animals.  Animals, 
therefore,  cannot  feed  on  chemical  compounds  of  the 
mineral  kingdom ;  they  must  have  organic  food  which 
easily  runs  into  decomposition  ;  they  must  feed  on  the 
vegetable  kingdom. 

Animals  are  distinguished  from  vegetables  by  inces- 
sant decay  in  every  tissue — a  decay  which  is  proportion- 
al to  animal  activity.  This  incessant  decay  necessitates 
incessant  rej^air,  so  that  the  animal  body  has  been  lik- 
ened to  a  temple  on  which  two  opposite  forces  are  at 
work  in  every  part,  the  one  tearing  down,  the  other  re- 
pairing the  breach  as  fast  as  made.  In  vegetables  no 
such  incessant  decay  has  ever  been  made  out.  If  it  ex- 
ists, it  must  be  very  trifling  in  comparison.  Protoplasm, 
it  is  true,  is  taken  up  from  the  older  parts  of  vegetables, 
and  these  parts  die  ;  but  the  protoplasm  does  not  seem 
to  decompose,  but  is  used  again  for  tissue-building. 
Thus  the  internal  activity  of  animals  is  of  two  kinds, 
tissue-destroying  and   tissue-building  ;    while   that   of 


PHYSICAL,   CHEMICAL,   AND   VITAL  FORCES.         189 

plants  seems  to  be,  principally,  at  least,  of  one  kind,  tis- 
sue-building. Animals  use  food  for  force  and  repair  and 
growth,  and  in  the  mature  animal  only  for  force  and 
repair.  Plants,  except  in  reproduction,  use  food  almost 
wholly  for  growth — they  never  stop  growing. 

I^ow,  the  food  of  animals  is  of  two  kinds,  amyloids 
and  albuminoids.  The  carnivora  feed  entirely  on  albu- 
minoids ;  herbivora  on  both  amyloids  and  albuminoids. 
All  this  food  comes  from  the  vegetable  kingdom,  di- 
rectly in  the  case  of  herbivora,  indirectly  in  the  case  of 
carnivora.  Animals  cannot  make  organic  matter.  Now, 
the  tissues  of  animals  are  wholly  albuminoid.  It  is  ob- 
vious, therefore,  that  for  the  repair  of  the  tissues  the 
food  must  be  albuminoid.  The  amyloid  food,  therefoi'e 
(and,  as  we  shall  see  in  carnivora,  much  of  the  albumi- 
noid), must  be  used  wholly  for  force.  As  coal  or  wood, 
burned  in  a  steam-engine,  changes  chemical  into  me- 
chanical energy,  so  food,  in  excess  of  what  is  used  for 
repair,  is  burned  up  to  produce  animal  activity.  Let  us 
trace  more  accurately  the  origin  of  animal  force  by  ex- 
amples. 

10.  Caenivoka. — The  food  of  carnivora  is  entirely 
albuminoid.  The  idea  of  the  older  physiologists,  in  re- 
gard to  the  use  of  this  food,  seems  to  have  been  as  fol- 
lows :  Albuminoid  matter  is  exceedingly  unstable  ;  it  is 
matter  raised,  with  much  difiSculty  and  against  chemical 
forces,  high,  and  delicately  balanced  on  a  pinnacle,  in  a 


£90  'T^E  CONSEKVATION  OF  ENEEGY. 

state  of  unstable  equilibrium,  for  a  brief  time,  and  then 
rushes  down  again  into  the  mineral  kingdom.  The  ani' 
mal  tissues,  being  formed  of  albuminoid  matter,  are 
short-lived  ;  the  parts  are  constantly  dying  and  decom- 
posing ;  the  law  of  death  necessitates  the  law  of  repro- 
duction ;  decomposition  necessitates  repair,  and  there- 
fore food  for  repair.  Eut  the  force  by  which  repair  is 
effected  was  for  them,  and  for  many  physiologists  now, 
underived,  innate.  But  the  doctrine  maintained  by  me 
in  the  paper  referred  to  is,  that  the  decomposition  of  the 
tissues  creates  not  only  the  necessity,  but  also  the  force, 
of  repair. 

Suppose,  in  the  first  place,  a  carnivorous  animal  uses 
just  enough  food  to  rej)air  the  tissues,  and  no  more — 
say  an  ounce.  Then  I  say  the  ounce  of  tissue  decayed 
not  only  necessitates  the  ounce  of  albuminous  food  for 
repair,  but  the  decomposition  sets  free  the  force  by  which 
the  repair  is  effected.  But  it  will  be  perhaps  objected 
that  the  force  would  all  be  consumed  in  repair,  and  none 
left  for  animal  activity  of  all  kinds.  I  answer  :  it  would 
not  all  be  used  up  in  repair,  for,  the  food  being  already 
albuminoid,  there  is  probably  little  expenditure  of  force 
necessary  to  change  it  into  tissue  ;  while,  on  the  other 
hand,  the  force  generated  by  the  decomposition  of  tissue 
into  CO2,  H2O,  and  urea,  is  very  great — the  ascensive 
change  is  small,  the  descensive  change  is  great.  The 
decomposition  of  one  ounce  of  albuminous  tissue  into 


PHYSICAL,  CHEMICAL,  AND  VITAL  FORCES.         191 

CO2,  HjO,  and  urea,  would  therefore  create  force  suffi- 
cient not  only  to  change  one  ounce  of  albuminous  mat- 
ter into  tissue,  but  also  leave  a  considerable  amount  for 
animal  activities  of  all  kinds.  A  certain  quantity  of 
matter,  running  down  from  plane  No.  4  to  plane  Xo.  2, 
creates  force  enough  not  only  to  move  the  same  quanti- 
ty of  matter  about  on  plane  Xo,  4,  but  also  to  do  much 
other  work  besides.  It  is  probable,  however,  that  the 
wants  of  animal  activity  are  so  immediate  and  urgent 
that,  under  these  conditions,  much  food  would  be  burned 
for  this  purpose,  and  would  not  reach  the  tissues,  and 
the  tissues  would  be  imperfectly  repaired,  and  would 
therefore  waste. 

Take,  next,  the  carnivorous  animal  full  fed.  In  this 
case  there  can  be  no  doubt  that,  while  a  portion  of  the 
food  goes  to  repair  the  tissues,  by  far  the  larger  por- 
tion is  consumed  in  the  blood,  and  passes  away  partly 
as  CO2  and  H^O  through  the  lungs,  and  partly  as  urea 
through  the  kidneys.  This  part  is  used,  and  can  be  of 
use  only,  to  create  force.  The  food  of  carnivora,  there- 
fore, goes  partly  to  tissue-building,  and  partly  to  create 
heat  and  force.  The  force  of  carnivorous  animals  is  de- 
rived partly  from  decomposing  tissues  and  partly  from 
food-excess  consumed  in  the  blood. 

11.  Heebivoka. — The  food  of  herbivora  and  of  man 
is  mixed — partly  albuminoid  and  partly  amyloid.  In 
man,  doubtless,  the  albuminoids  are  usually  in  excess  of 


192  THE  CONSERVATION  OF  ENERGY. 

wliat  is  required  for  tissue-building ;  but  in  berbivora, 
probably,  tbe  albuminoids  are  not  in  excess  of  tbe  re- 
quirements of  the  decomposing  tissues.  In  this  case, 
therefore,  the  whole  of  the  albuminoids  is  used  for  tis- 
sue-making, and  the  whole  of  the  amyloids  for  force- 
making.  In  this  class,  therefore,  these  two  classes  of 
food  may  be  called  tissue-food  and  force-food.  The  force 
of  these  animals,  therefore,  is  derived  partly  from  the 
decomposition  of  the  tissues,  but  principally  from  the 
decomj^osition  and  combustion  of  the  amyloids  and 
fats. 

Some  physiologists  speak  of  the  amyloid  and  fat 
food  as  being  burned  to  keep  up  the  animal  heat ;  but 
it  is  evident  that  the  prime  object  in  the  body,  as  in  the 
steam-engine,  is  not  heat,  but  force.  Heat  is  a  mere 
condition  and  perhaps  a  necessary  concomitant  of  the 
change,  but  evidently  not  the  prime  object.  In  tropical 
regions  the  heat  is  not  wanted.  In  the  steam-engine, 
chemical  energy  is  first  changed  into  heat,  and  heat  into 
mechanical  energy ;  in  the  body  the  change  is,  proba- 
bly, much  of  it  direct,  and  not  through  the  intermedia- 
tion of  heat. 

12.  We  see  at  once,  from  the  above,  why  it  is  that 
plants  cannot  feed  on  elements,  viz.,  because  their  food 
must  be  decomposed  in  order  to  create  the  organic 
matter  out  of  which  all  organisms  are  built.  This 
elevation  of  matter,  which  takes  place  in  the  green 


PHYSICAL,   CHEMICAL,   AND  VITAL  FOKCES.  I93 

leaves  of  plants,  is  the  starting-point  of  life ;  upon  it 
alone  is  based  the  possibility  of  the  existence  of  the 
organic  kingdom.  The  running  down  of  the  matter 
there  raised  determines  the  vital  phenomena  of  germi- 
nation, of  pale  plants,  and  even  of  some  of  the  vital 
phenomena  of  green  plants,  and  all  the  vital  phenom- 
ena of  the  animal  kingdom.  The  stability  of  chem- 
ical compounds,  usable  as  plant-food,  is  such  that  a 
peculiar  contrivance  and  peculiar  conditions  found  only 
in  the  green  leaves  of  plants  are  necessary  for  their  de- 
composition. "We  see,  therefore,  also,  why  animals  as 
well  as  pale  plants  cannot  feed  on  mineral  matter. 

We  easily  see  also  why  the  animal  activity  of  carniv- 
ora  is  greater  than  that  of  herbivora,  for  the  amount 
of  force  necessary  for  the  assimilation  of  their  albumi- 
noid food  is  small,  and  therefore  a  larger  amount  is  left 
over  for  animal  activity.  Their  food  is  already  on  plane 
No.  4  ;  assimilation,  therefore,  is  little  more  than  a  shift- 
ing on  the  plane  No.  4  from  a  liquid  to  a  solid  condition 
— from  liquid  albuminoid  of  the  blood  to  solid  albumi- 
noid of  the  tissues. 

We  see  also  why  the  internal  activity  of  plants  may 
conceivably  be  only  of  one  kind  ;  for,  drawing  their 
force  from  the  sun,  tissue-making  is  not  necessarily  de- 
pendent on  tissue-decay.  While,  on  the  other  hand, 
the  internal  activity  of  animals  must  be  of  two  kinds, 
decay  and  repair  ;  for  animals  always  draw  a  portion  of 


19i  THE  CONSEEVATION  OF  ENEKGY. 

tlieii'  force,  and  starving  animals  tlie  whole  of  their  forct;, 
from  decaying  tissue. 

13.  There  are  several  general  thoughts  suggested  by 
this  subject,  which  I  wish  to  present  in  conclusion  : 

a.  We  have  said  there  are  four  planes  of  matter 
raised  one  above  the  other  :  1.  Elements  ;  2.  Chemical 
compounds ;  3.  Yegetables  ;  4.  Animals.  Their  rela- 
tive position  is  truly  represented  thus  : 

4.  Animals. 

3.  Plants. 

2.   Chemical  comjyounds, 

1.  Elements. 

I^ow,  there  are  also  four  planes  of  force  similarly  r<> 
Jated  to  each  other,  viz.,  physical  force,  chemical  force, 
vitality,  and  will.  On  the  first  plane  of  matter  operates 
physical  force  only ;  for  chemical  force  immediately 
raises  matter  into  the  second  plane.  On  the  second 
plane  operates,  in  addition  to  physical,  also  chemical 
force.  On  the  third  plane  operates,  in  addition  to  phys- 
ical and  chemical,  also  vital  force.  On  the  fourth  plane, 
in  addition  to  physical,  chemical,  and  vital,  also  the  force 
characteristic  of  animals,  viz.,  will.*     With  each  eleva- 

*  I  might  add  still  another  plane  and  another  force,  viz.,  the  human 
plane,  on  which  operate,  in  addition  to  all  the  lower  forces,  also  free-will 
and  reason.  I  do  not  speak  of  these,  only  because  they  lie  beyond  th<J 
present  ken  of  inductive  s-cience. 


PHYSICAL,   CHEMICAL,  AND   VITAL  FORCES.  195 

tiou  there  is  a  peculiar  force  added  to  tlie  already  exist- 
in;^,  and  a  peculiar  group  of  phenomena  is  the  result. 
As  matter  only  rises  step  by  step  from  plane  to  plane, 
and  never  two  steps  at  a  time,  so  also  force,  in  its  trans- 
formation into  higher  forms  of  force,  rises  only  step  by 
step.  Physical  force  does  not  become  vital  except 
through  chemical  force,  and  chemical  force  does  not  be- 
come will  except  through  vital  force. 

Again,  we  have  compared  the  various  grades  of  mat- 
ter, not  to  a  gradually  rising  inclined  plane,  but  to  suc- 
cessive planes  raised  one  above  the  other.  There  are, 
no  doubt,  some  intermediate  conditions  :  but,  as  a  broad, 
general  fact,  the  changes  from  plane  to  plane  are  sud- 
den. !Now,  the  same  is  true  also  of  the  forces  operating 
on  these  planes — of  the  different  grades  of  force,  and 
their  corresponding  groups  of  phenomena.  The  change 
from  one  grade  to  another,  as  from  physical  to  chemical, 
or  from  chemical  to  vital,  is  not,  as  far  as  we  can  see,  by 
sliding  scale,  but  suddenly.  The  groups  of  phenomena 
which  we  call  physical,  chemical,  vital,  animal,  rational, 
and  moral,  do  not  merge  into  each  other  by  insensible 
gradations.  In  the  ascensive  scale  of  forces,  in  the  evo- 
lution of  the  higher  forces  from  the  lower,  there  are 
places  of  rapid,  paroxysmal  change. 

h.  Yital  force  is  transformed  into  physical  and  chem 
ical  forces  ;  but  it  is  not  on  that  account  identical  with 
physical  and  chemical  force,  and  therefore  we  ought  not, 


[96  THE  CONSERVATION  OF  ENEKGY. 

as  some  would  have  lis,  discard  tlie  term  vital  force. 
There  are  two  opposite  errors  on  this  subject :  one  is 
the  old  error  of  regarding  vital  force  as  something  in- 
nate, underived,  having  no  relation  to  the  other  forces 
of  JSTature  ;  the  other  is  the  new  error  of  regarding  the 
forces  of  the  living  body  as  nothing  but  ordinary  physi- 
cal and  chemical  forces,  and  therefore  insisting  that  the 
use  of  the  term  vital  force  is  absurd  and  injurious  to 
science.  The  old  error  is  still  prevalent  in  the  popular 
mind,  and  still  haunts  the  minds  of  many  physiologists; 
the  new  error  is  apparently  a  revulsion  from  the  other, 
and  is  therefore  common  among  the  most  advanced  sci- 
entific minds.  There  are  many  of  the  best  scientists 
who  ridicule  the  use  of  the  term  vital  force,  or  vitality, 
as  a  remnant  of  superstition  ;  and  yet  the  same  men  use 
the  words  gravity,  magnetic  force,  chemical  force,  phys- 
ical force,  etc.  Vital  force  is  not  imderived — is  not  un- 
related to  other  forces — is,  in  fact,  correlated  with  them  ; 
but  it  is  nevertheless  a  distinct  form  of  force,  far  more 
distinct  than  any  other  form,  unless  it  be  still  higher 
forms,  and  therefore  better  entitled  to  a  distinct  name 
than  any  lower  form.  Each  form  of  force  gives  rise  to 
a  peculiar  group  of  phenomena,  and  the  study  of  these 
to  a  peculiar  department  of  science.  Now,  the  group 
of  phenomena  called  vital  is  more  peculiar,  and  more 
different  from  other  groups,  than  these  are  from  each 
other ;  and  the  science  of  physiology  is  a  more  distinct 


PHYSICAL,  CHEMICAL,   AND   VITAL  FOECES.  J97 

department  than  eitlier  physics  or  chemistry  ;  and  there- 
fore the  form  of  force  which  determines  these  phenom- 
ena is  more  distinct,  and  better  entitled  to  a  distinct 
name,  than  either  physical  or  chemical  forces.  De  Can- 
dolle,  in  a  recent  paper,*  suggests  the  term  vital  move- 
ment instead  of  vital  force ;  but  can  we  conceive  of  move- 
ment without  force  ?  And,  if  the  movement  is  peculiar, 
60  also  is  the  form  of  force. 

G.  Yital  is  transformed  physical  and  chemical  forces  ; 
true,  but  the  necessary  and  very  peculiar  condition  of 
this  transformation  is  the  previous  existence  then  and 
there  of  living  matter.  There  is  something  so  wonder- 
ful in  this  peculiarity  of  vital  force  that  I  must  dwell  on 
it  a  little. 

Elements  brought  in  contact  with  each  other  under 
certain  physical  conditions — perhaps  heat  or  electricity 
— unite  and  rise  into  the  second  plane,  i.  e.,  of  chemical 
compounds ;  so  also  several  elements,  C,  H,  O,  and  N, 
etc.,  brought  in  contact  with  each  other  under  certain 
physical  or  chemical  conditions,  such  as  light,  nascency, 
etc.,  unite  and  rise  into  plane  ~So.  3,  i.  e.,  form  organic 
matter.  In  both  cases  there  is  chemical  union  under 
certain  physical  conditions ;  but  in  the  latter  there  is 
one  unique  condition,  viz.,  the  previous  existence  then 
and  there  of  organic  matter,  under  the  guidance  of 
which  the  transformation  of  matter  takes  place.     In  a 

*  Archives  des  Sciences,  vol.  xlv.,  p.  345,  December,  1872. 


198  '^ilE  CONSEEVATION  OF  ENERGY. 

word,  organic  matter  is  necessary  to  produce  organic 
matter ;  there  is  here  a  law  of  like  producing  like — • 
there  is  an  assimilation  of  matter. 

Again,  physical  force  changes  into  other  forms  of 
physical  force,  or  into  chemical  force,  under  certain 
physical  conditions  ;  so  also  physical  and  chemical  forces 
are  changed  into  vital  force  under  certain  physical  con- 
ditions. But,  in  addition,  there  is  one  altogether  unique 
condition  of  the  latter  change,  viz.,  the  previous  exist- 
ence then  and  there  of  vital  force.  Here,  again,  like 
produces  like  —  here,  again,  there  is  assimilation  of 
force. 

This  law  of  like  producing  like — this  law  of  assimi- 
lation of  matter  and  force — runs  throughout  all  vital 
phenomena,  even  to  the  minutest  details.  It  is  a  uni- 
versal law  of  generation,  and  determines  the  existence 
of  species  ;  it  is  the  law  of  formation  of  organic  matter 
and  organic  force  ;  it  determines  all  the  varieties  of  or- 
ganic matter  which  we  call  tissues  and  organs,  and  all 
the  varieties  of  organic  force  which  we  call  functions. 
The  same  nutrient  pabulum,  endowed  with  the  same 
properties  and  powers,  carried  to  all  parts  of  a  complex 
organism  by  this  wonderful  law  of  like  producing  like, 
is  changed  into  the  most  various  forms  and  endowed 
with  the  most  various  powers.  There  are  certainly 
limits  and  exceptions  to  this  law,  however ;  otherwise 
differentiation  of  tissues,  organs,  and  functions,  could 


PHYSICAL,   CHEMICAL,   AND  VITAL  FOKCES.  199 

not  take  place  iu  embryonic  development  ;  but  tlie 
limits  and  exceptions  are  tbemselves  subject  to  a  law 
even  more  wonderful  tlian  tbe  law  of  like  producing 
like  itself,  viz.,  tbe  law  of  evolution.  There  is  in  all 
organic  nature,  wlietlier  organic  kingdom,  organic  in- 
dividual, or  organic  tissues,  a  law  of  variation,  strongest 
in  the  early  stages,  limited  very  strictly  by  another  law 
— the  law  of  inheritance,  of  like  producing  like. 

d.  "VVe  have  seen  that  all  development  takes  place  at 
the  expense  of  decay — all  elevation  of  one  thing,  in  one 
place,  at  the  expense  of  corresponding  running  doAvn 
of  something  else  in  another  place.  Force  is  only  trans- 
ferred and  transformed.  The  plant  draws  its  force  from 
the  sun,  and  therefore  what  the  plant  gains  the  sun 
loses.  Animals  draw  from  plants,  and  therefore  what 
the  anifnal  kingdom  gains  the  vegetable  kingdom  loses. 
Again,  an  ^gg^  a  seed,  or  a  chrysalis,  developing  to  a 
higher  condition,  and  yet  taking  nothing  cib  extra,  must 
lose  weight.  Some  part  must  run  down,  in  order  that 
the  remainder  should  be  raised  to  a  higher  condition. 
The  amount  of  evolution  is  measured  by  the  loss  of 
weight.  By  the  law  of  conservation  of  force,  it  is  in- 
conceivable that  it  should  be  otherwise.  Evidently, 
therefore,  in  the  universe,  taken  as  a  whole,  evolution 
of  one  part  must  be  at  the  expense  of  some  other  part. 
The  evolution  or  development  of  the  whole  cosmos — of 
the  whole  universe  of  matter — as  a  unit,  by  forces  with 


200        THE  CONSEEVATION  OF  ENEEGY. 

in  itself,  according  to  tlie  doctrine  of  conservation  of 
force,  is  inconceivable.  If  there  be  any  such  evolution, 
at  all  comparable  with  any  known  form  of  evolution,  it 
can  only  take  place  by  a  constant  increase  of  the  whole 
sum  of  energy,  i.  e.,  by  a  constant  influx  of  divine  en- 
ergy— for  the  same  quantity  of  matter  in  a  higher  con- 
dition must  embody  a  greater  amount  of  energy. 

e.  Finally,  as  organic  matter  is  so  much  matter  tak- 
en from  the  common  fund  of  matter  of  earth  and  air, 
embodied  for  a  brief  space,  to  be  again  by  death  and 
decomposition  returned  to  that  common  fund,  so  also  it 
would  seem  that  the  organic  forces  of  the  living  bodies 
of  plants  and  animals  may  be  regarded  as  so  much  force 
drawn  from  the  common  fund  of  physical  and  chemical 
forces,  to  be  again  all  refunded  by  death  and  decompo- 
sition. Yes,  by  decomposition  ;  we  can  understand  this. 
But  death  !  can  we  detect  any  thing  returned  by  simple 
death  ?  What  is  the  nature  of  the  diiference  between 
the  living  organism  and  a  dead  organism  ?  ^Ye  can  de- 
tect none,  physical  or  chemical.  All  the  physical  and 
chemical  forces  withdrawn  from  the  common  fund  of 
JSTature,  and  embodied  in  the  living  organism,  seem  to 
be  still  embodied  in  the  dead  until  little  by  little  it  is 
returned  by  decomposition.  Yet  the  difference  is  im- 
mense, is  inconceivably  great.  What  is  the  nature  of 
this  difference  expressed  in  the  formula  of  material  sci- 
ence ?    What  is  it  that  is  gone,  and  whither  is  it  gone  ? 


PHYSICAL,  CHEMICAL,  AND  VITAL  FORCES.  201 

There  is  something  here  which  science  cannot  yet  under- 
stand. Yet  it  is  just  this  loss  which  takes  place  in  death, 
and  before  decomposition,  which  is  in  the  highest  sense 
vital  force. 

Let  no  one  from  the  above  views,  or  from  similar 
views  expressed  by  others,  draw  hasty  conclusions  in 
favor  of  a  pure  materialism.  Force  and  matter,  or 
spirit  and  matter,  or  God  and  Nature,  these  are  the  op- 
posite poles  of  philosophy — they  are  the  opposite  poles 
of  thought.  There  is  no  clear  thinking  without  them. 
Not  only  religion  and  virtue,  but  science  and  philosophy, 
cannot  even  exist  without  them.  The  belief  in  spirit, 
like  the  belief  in  matter,  rests  on  its  own  basis  of  phe- 
nomena. The  true  domain  of  philosophy  is  to  reconcile 
these  wuth  each  other. 


CORRELATION   OF   I^ERVOUS   AND   MENTAL 
FORCES. 

By  ALEXANDER  BAIN",  LL.  D., 

PROFESSOR   OF   LOGIC   AND    MENTAL    PHILOSOPnY    IN    THK 
ONIVERSITT    OF   ABERDEEN. 


THE   CORRELATION  OF  NERVOUS   AND 
MENTAL  FORCES. 


The  doctrine  called  the  correlation,  persistence, 
equivalence,  transmiitability,  indestructibility  of  force, 
or  tlie  conservation  of  energy,  is  a  generality  of  sucli 
compass  that  no  single  form  of  words  seems  capable  of 
fully  expressing  it ;  and  different  persons  may  prefer 
different  statements  of  it.  My  understanding  of  the 
doctrine  is,  that  there  are  five  chief  powers  or  forces 
in  I^ature  :  one  mechanical^  or  molar,  the  momentum 
of  moving  matter ;  the  others  molecular,  or  embodied 
in  the  molecules,  also  supposed  in  motion — these  are, 
heat,  light,  chemical  force,  electricity.  To  these  powers, 
which  are  unquestionable  and  distinct,  it  is  usual  to  add 
vital  force,  of  which,  however,  it  is  difiicult  to  speak  as 
a  whole ;  but  one  member  of  our  vital  energies,  the 
nerve-force,  allied  to  electricity,  fully  deserves  to  rank 
in  the  correlation. 

Taking  the  one  mechanical  force,  and  those  three  of 


206  TILE   CONSEliVATlOJN    OF   ENERGY. 

the  molecular  named  heat,  chemical  force,  electricity, 
there  has  now  been  established  a  definite  rate  of  com- 
mutation, or  exchange,  when  any  one  passes  into  any 
other.  The  mechanical  equivalent  of  heat,  the  772  foot- 
pounds of  Joule,  expresses  the  rate  of  exchange  between 
mechanical  momentum  and  heat :  the  equivalent  or  ex- 
change of  heat  and  chemical  force  is  given  (through  the 
researches  of  Andrews  and  others)  in  the  figures  ex- 
pressing the  heat  of  combinations ;  for  example,  one 
pound  of  carbon  burnt  evolves  heat  enough  to  raise 
8,080  pounds  of  water  one  degree,  C.  The  combination 
of  these  to  equivalents  would  show  that  the  consump- 
tion of  half  a  pound  of  carbon  would  raise  a  man  of 
average  weight  to  the  highest  summit  of  the  Himalayas. 

It  is  an  essential  part  of  the  doctrine,  that  force  is 
never  absolutely  created,  and  never  absolutely  destroyed, 
but  merely  transmuted  in  form  or  manifestation. 

As  applied  to  living  bodies,  the  following  are  the 
usual  positions.  In  the  growth  of  plants,  the  forces  of 
the  solar  ray — heat  and  light — are  expended  in  decom- 
posing (or  deoxidizing)  carbonic  acid  and  water,  and  in 
building  up  the  living  tissues  from  the  liberated  carbon 
and  the  other  elements ;  all  which  force  is  given  up 
when  these  tissues  are  consumed,  either  as  fuel  in  ordi- 
nary combustion,  or  as  food  in  animal  combustion. 

It  is  this  animal  combustion  of  the  matter  of  plants, 
and  of  animals  (fed  on  plants) — namely,  the  reoxida- 


NERVOUS  AND  MENTAL  EOKCES.        207 

tion  of  carbon,  hydrogen,  etc. — that  yields  all  the  man- 
ifestations of  power  in  the  animal  frame.  And,  in  par- 
ticular, it  maintains  (1)  a  certain  warmth  or  tempera- 
ture of  the  whole  mass,  against  the  cooling  power  of 
surrounding  space ;  it  maintains  (2)  mechanical  energy, 
as  muscular  power ;  and  it  maintains  (3)  nervous  power, 
or  a  certain  flow  of  the  influence  circulating  through  the 
nerves,  which  circulation  of  influence,  besides  reacting 
on  the  other  animal  processes — muscular,  glandular,  etc. 
— has  for  its  distinguishing  concomitant  the  mixd. 

The  extension  of  the  correlation  of  force  to  mind, 
if  at  all  competent,  must  be  made  through  the  nerve- 
forcCy  a  genuine  member  of  the  correlated  group.  Very 
serious  difficulties  beset  the  proposal,  but  they  are  not 
insuperable. 

The  history  of  the  doctrines  relating  to  mind,  as 
connected  with  body,  is  in  the  highest  degree  curious 
and  instructive,  but,  for  the  purpose  of  the  present  pa- 
per, we  shall  notice  only  certain  leading  stages  of  the 
speculation.* 

Not  the  least  important  position  is  the  Aristotelian  ; 
a  position  in  some  respects  sounder  than  what  followed 
and  grew  out  of  it.  In  Aristotle,  we  have  a  kind  of 
gradation  from  the  life  of  plants  to  the  highest  form  of 

*  For  the  fuller  elaboration  of  the  point  here  referred  to,  see  Chapter 
VII.,  Professor  Bain's  "  Mind  and  Body  " — an  earlier  volume  in  the  pres- 
ent series. 

10 


208  THE  CONSEKVATION  OF  ENERGY. 

human  intelligence.  In  the  following  diagram,  tlie  con- 
tinuous lines  may  represent  tlie  material  substance,  and 
tlie  dotted  lines  the  immaterial : 

A.  Soul  of  Plants. 
Without  consciousness. 


B.  Animal  Soul. 
Body  and  mind  inseparable. 

C.  Euman  Soul— li^ova — Intellect. 

I.  Passive  intellect. 

Body  and  mind  inseparable. 

II.  Active  intellect — cognition  of  the  highest  principles. 
Pure  form ;  detached  from  matter ;  the  prime  mover  of 

all ;  immortal. 

All  the  phases  of  life  and  mind  are  inseparably  in- 
terwoven with  the  body  (which  inseparability  is  Aris- 
totle's definition  of  the  soul)  except  the  last,  the  active 
nous,  or  intellect,  which  is  detached  from  corporeal  mat- 
ter, self-subsisting,  the  essence  of  Deity,  and  an  immor- 
tal substance,  although  the  immortality  is  not  personal 
to  the  individual.  (The  immateriality  of  this  higher 
intellectual  agent  was  net,  however,  that  thorough-go- 
ins:  negation  of  all  material  attributes  which  we  now 
understand  by  the  word  "  immaterial.")  How  such  a 
self-subsisting  and  purely  spiritual  soul  could  hold  com- 
munication with  the  body-leagued  souls,  Aristotle  was 
at  ti  loss  to  say — the  difficulty  reappeared  after  him,  and 


NERVOUS  AND  MENTAL  FOECES.         209 

has  never  been  got  over.  Tliat  there  should  be  an 
agency  totally  apart  from,  and  entirely  transcending, 
any  known  powers  of  inert  matter,  involves  no  difficul- 
ty— for  who  is  to  limit  the  possibilities  of  existence  ? 
The  perplexity  arises  only  when  this  radically  new  and 
superior  principle  is  made  to  be,  as  it  were,  off  and  on 
with  the  material  principle  ;  performing  some  of  its 
functions  in  pure  isolation,  and  others  of  an  analogous 
kind  by  the  aid  of  the  lower  principle.  The  diiference 
between  the  active  and  the  passive  reason  of  Aristotle 
is  a  mere  difference  of  gradation  ;  the  supporting 
agencies  assumed  by  him  are  a  total  contrast  in  kind 
— wide  as  the  poles  asunder.  There  is  no  breach  of 
continuity  in  the  phenomena,  there  is  an  impassable 
chasm  between  their  respective  foundations. 

Fifteen  centuries  after  Aristotle,  we  reach  what  may 
be  called  the  modern  settlement  of  the  relations  of 
mind  and  body,  effected  by  Thomas  Aquinas.  He  ex- 
tended the  domain  of  the  independent  immaterial  prin- 
ciple from  the  highest  intellectual  soul  of  Aristotle  to 
all  the  three  souls  recognized  by  him — the  vegetable  or 
plant  soul  (without  consciousness),  the  animal  soul  (with 
consciousness),  and  the  intellect  throughout.  The  two 
lower  souls — the  vegetable  and  the  animal — need  the 
cooperation  of  the  body  in  this  life  ;  the  intellect  works 
without  any  bodily  organ,  except  that  it  makes  use  of 
the  perceptions  of  the  senses. 


210  THE  CONSEfiVATION  OF  ENEEGY. 

A.    Vegetable  or  Ifutritke  Soul. 
Incorporates  an  immaterial  part,  although  unconscioas. 

B.  Animal  Soul. 
Has  an  immaterial  part,  with  consciousness. 

C.  Intellect. 
Purely  immaterial. 

The  animal  soul,  B,  contains  sensation,  appetite,  and 
emotion,  and  is  a  mixed  or  two-sided  entity ;  but  the 
intellect,  0,  is  a  pm-ely  one-sided  entity,  the  immaterial. 
This  does  not  relieve  om-  perplexities  ;  the  phenomena 
are  still  generically  allied  and  continuous  —  sensation 
passes  into  intellect  without  any  breach  of  continuity  ; 
but  as  regards  the  agencies,  the  transition  from  a  mixed 
or  united  material  and  immaterial  substance  to  an  imma- 
terial substance  apart,  is  a  transition  to  a  differently  con- 
stituted world,  to  a  transcendental  sphere  of  existence. " 

The  settlement  of  Aquinas  governed  all  the  schools 
and  all  the  religious  creeds,  until  quite  recent  times  ;  it 
is,  for  example,  substantially  the  view  of  Bishop  Butler. 
At  the  instance  of  modern  physiology,  however,  it  has 
undergone  modifications.  The  dependence  of  purely 
intellectual  operations,  as  memory,  upon  the  material 
processes,  has  been  reluctantly  admitted  by  the  partisans 
of  an  immaterial  principle ;  an  admission  incompatible 
with  the .  isolation  of  the  intellect  in  Aristotle  and  in 
Aquinas.     This  more  thorough-going  connection  of  the 


NERVOUS  AND  MENTAL  FOECES.        211 

mental  and  the  physical  has  led  to  a  new  form  of  ex- 
pressing the  relationship,  which  is  nearer  the  truth, 
without  being,  in  my  judgment,  quite  accurate.  It  ia 
now  often  said  the  mind  and  the  hodij  act  ujton  each 
other  ;  that  neither  is  allowed,  so  to  speak,  to  pursue  its 
course  alone — there  is  a  constant  interference,  a  mutual 
influence  between  the  two.  This  view  is  liable  to  the 
following  objections : 

1.  In  the  first  place,  it  assumes  that  we  are  entitled 
to  speak  of  mind  apart  from  body,  and  to  affirm  its 
powers  and  properties  in  that  separate  capacity.  But 
of  mind  apart  from  body  we  have  no  direct  experience, 
and  absolutely  no  knowledge.  The  wind  may  act  upon 
the  sea,  and  the  waves  may  react  upon  the  wind ;  but 
the  agents  are  known  in  separation — they  are  seen  to 
exist  apart  before  the  shock  of  collision  ;  but  we  are  not 
permitted  to  see  a  mind  acting  apart  from  its  material 
companion. 

2.  In  the  second  place,  we  have  every  reason  for  be- 
lieving that  there  is  an  unbroken  material  succession, 
side  by  side  with  all  our  mental  processes.  From  the 
ingress  of  a  sensation,  to  the  outgoing  responses  in  ac- 
tion, the  mental  succession  is  not  for  an  instant  dis- 
severed from  a  physical  succession.  A  new  prospect 
bursts  upon  the  view ;  there  is  a  mental  result  of  sen- 
sations, emotion,  thought,  terminating  in  outward  dis- 
plays  of  speech   or  gesture.     Parallel  to  this   mental 


212  THE  CONSERVATION  OF  ENEEGY. 

series  is  the  physical  series  of  facts,  the  successive  agita- 
tion of  the  physical  organs,  called  the  eye,  the  retina, 
the  optic  nerve,  optic  centres,  cerebral  hemispheres, 
outgoing  nerves,  muscles,  etc.  There  is  an  unbroken 
physical  circle  of  effects,  maintained  while  we  go  the 
round  of  the  mental  circle  of  sensation,  emotion,  and 
thought.  It  would  be  incompatible  with  every  thing 
we  know  of  the  cerebral  action  to  suppose  that  the  phys. 
ical  chain  ends  abruptly  in  a  physical  void,  occupied  by 
an  immaterial  substance ;  which  immaterial  substance, 
after  working  alone,  imparts  its  results  to  the  other  edge 
of  the  physical  break,  and  determines  the  active  response 
• — two  shores  of  the  material  with  an  intervening  ocean  of 
the  immaterial.  There  is,  in  fact,  no  rupture  of  nervous 
continuity.  The  only  tenable  supposition  is,  that  men- 
tal and  physical  proceed  together,  as  individual  twins. 
When,  therefore,  we  speak  of  a  mental  cause,  a  mental 
agency,  we  have  always  a  two-sided  cause ;  the  effect 
produced  is  not  the  effect  of  mind  alone,  but  of  mind  in 
company  with  body.  That  mind  should  have  operated 
on  the  body,  is  as  much  as  to  say  that  a  two-sided  phe- 
nomenon, one  side  being  bodily,  can  influence  the  body  ; 
it  is,  after  all,  body  acting  upon  body.  When  a  shock 
of  fear  paralyzes  digestion,  it  is  not  the  emotion  of  fear, 
in  the  abstract,  or  as  a  pure  mental  existence,  that  does 
thchann  ;  it  is  the  emotion  in  company  with  a  peculiarly 
excited  condition  of  the  brain  and  nervous  system  ;  alid 


NEEVOUS  AND  MENTAL  FOECES,         213 

it  is  tliis  condition  of  tlie  brain  that  deranges  tlie 
stomacli.  When  physical  nourishment,  or  physical  stim- 
ulant, acting  through  the  blood,  quiets  the  mental  irri- 
tation,  and  restores  a  cheerful  tone,  it  is  not  a  bodily 
fact  causing  a  mental  fact  by  a  direct  line  of  causation  : 
the  nourishment  and  the  stimulus  determine  the  circula- 
tion of  blood  to  the  brain,  give  a  new  direction  to  the 
nerve-currents,  and  the  mental  condition  corresponding 
to  this  particular  mode  of  cerebral  action  henceforth 
manifests  itself.  The  line  of  mental  sequence  is  thus, 
not  mind  causing  body,  and  body  causing  mind,  but 
mind-body  giving  birth  to  mind-body;  a  much  more 
intelligible  position.  For  this  double  or  conjoint  causa- 
tion, we  can  produce  evidence ;  for  the  single-handed 
causation  we  have  no  evidence. 

If  it  were  not  my  peculiar  province  to  endeavor  to 
clear  up  the  specially  metaphysical  difficulties  of  the 
relationship  of  mind  and  body,  I  would  pass  over  what 
is  to  me  the  most  puzzling  circumstance  of  the  relation- 
ship, and  indeed  the  only  real  difficulty  in  the  question. 

I  say  the  real  difficulty,  for  factitious  difficulties  in 
abundance  have  been  made  out  of  the  subject.  It  is 
made  a  mystery  how  mental  functions  and  bodily  func- 
tions should  be  allied  together  at  all.  That,  however, 
is  no  business  of  ours;  we  accept  this  alliance,  as  we 
do  any  other  alliance,  such  as  gravity  with  inert  matter, 
01-  lio-ht  with  heat.     As  a  fact  of  the  universe,  the  union 


214  THE  CONSERVATION   OF  ENERGY. 

is,  properly  speaking,  just  as  acceptable,  and  as  intelli- 
gible, as  tlie  separation  would  be,  if  that  were  the  fact. 
The  real  difficulty  is  quite  another  thing. 

What  I  have  in  view  is  this  :  when  I  speak  of  mind 
as  allied  with  body — with  a  brain  and  its  nerve-currenta 
— I  can  scarcely  avoid  localizing  the  mind,  giving  it  a 
local  habitation.  I  am  thereupon  asked  to  explain 
what  always  puzzled  the  schoolmen,  namely,  whether 
the  mind  is  all  in  every  part,  or  only  all  in  the  whole ; 
whether  in  tapping  any  point  I  may  come  at  conscious- 
ness, or  whether  the  whole  mechanism  is  wanted  for 
tlie  smallest  portion  of  consciousness.  One  might  per- 
haps turn  the  question  by  the  analogy  of  the  telegraph 
wire,  or  the  electric  circuit,  and  say  that  a  complete 
circle  of  action  is  necessary  to  any  mental  manifestation ; 
which  is  probably  true.  But  this  does  not  meet  the 
case.  The  fact  is  that,  all  this  time  we  are  speaking  of 
nerves  and  wires,  we  are  not  speaking  of  mind,  proper- 
ly so  called,  at  all ;  we  are  putting  forward  physical 
facts  that  go  along  with  it,  but  these  physical  facts  are 
not  the  mental  fact,  and  they  even  preclude  us  from 
thinking  of  the  mental  fact.  We  are  in  this  fix  :  men- 
tal states  and  bodily  states  are  utterly  contrasted  ;  they 
cannot  be  compared,  they  have  nothing  in  common  ex- 
cept the  most  general  of  all  attributes,  degree,  and  order 
in  time  ;  when  engaged  with  one  we  must  be  oblivious 
of  all  that  distinguishes  the  other.     When  I  am  study 


NEKVOUS  AND  MENTAL  FORCES,         215 

ing  a  brain  and  nerve  communicating,  I  am  engrossed 
with  properties  exclusively  belonging  to  the  object  or 
material  world ;  I  am  at  that  moment  (except  by  very 
rapid  transitions  or  alternations)  unable  to  conceive  a 
truly  mental  fact,  my  truly  mental  consciousness.  Our 
mental  experience,  our  feelings  and  thoughts,  have  no 
extension,  no  place,  no  form  or  outline,  no  mechanical 
division  of  parts  ;  and  we  are  incapable  of  attending  to 
any  thing  mental  until  we  shut  off  the  view  of  all  that. 
Walking  in  the  country  in  spring,  our  mind  is  occupied 
with  the  foliage,  the  bloom,  and  the  grassy  meads,  all 
purely  objective  things  ;  we  are  suddenly  and  strongly 
arrested  by  the  odor  of  the  May-blossom  ;  we  give 
way  for  a  moment  to  the  sensation  of  sweetness  :  for 
that  moment  the  objective  regards  cease ;  we  think  of 
nothing  extended ;  we  are  in  a  state  where  extension 
has  no  footing ;  there  is,  to  us,  place  no  longer.  Such 
states  are  of  short  duration,  mere  fits,  glimpses ;  they 
are  constantly  shifted  and  alternated  with  object  states, 
but  while  they  last  and  have  their  full  power  we  are  in 
a  different  world ;  the  material  world  is  blotted  out, 
eclipsed,  for  the  instant  unthinkable.  These  subject- 
moments  are  studied  to  advantage  in  bursts  of  intense 
pleasure,  or  intense  pain,  in  fits  of  engrossed  reflection, 
especially  reflection  upon  mental  facts ;  but  they  are  sel- 
dom sustained  in  purity  beyond  a  very  short  interval ;  we 
are  constantly  returning  to  the  object-side  of  things— 


216  THE  CONSEEVATION   OF  ENEEGY. 

to  the  world  wliere  extension   and   place   have   their 
being. 

This,  then,  as  it  appears  to  me,  is  the  only  real  diffi- 
culty of  the  physical  and  mental  relationship.  There 
is  an  alliance  with  matter,  with  the  object,  or  extended 
world ;  but  the  thing  allied,  the  mind  proper,  has  it- 
self no  extension,  and  cannot  be  joined  in  local  union. 
Now,  we  have  no  form  of  language,  no  ftimiliar  analogy, 
suited  to  this  unique  conjunction ;  in  comj^arison  with 
all  ordinary  unions,  it  is  a  paradox  or  a  contradiction. 
We  understand  union  in  the  sense  of  local  connection  ; 
here  is  a  union  where  local  connection  is  irrelevant,  un- 
suitable, contradictory,  for  we  cannot  think  of  mind 
without  putting  ourselves  out  of  the  world  of  place. 
When,  as  in  pure  feeling — pleasure  or  pain — we  change 
to  the  subject  attitude  from  the  object  attitude,  we  have 
undergone  a  change  not  to  be  expressed  by  place ;  the 
fact  is  not  properly  described  by  the  transition  from  the 
external  to  the  internal^  for  that  is  still  a  change  in  the 
region  of  the  extended.  The  only  adequate  expression 
is  a  change  of  state:  a  change  from  the  state  of  the  ex- 
tended cognition  to  a  state  of  unextended  cognition. 
By  various  theologians,  heaven  has  been  sj^ohen  of  as 
not  a  place,  but  a  state  ;  and  this  is  the  only  phrase  that 
•  I  can  find  suitable  to  describe  the  vast,  though  familiar 
and  easy,  transition  from  the  material  or  extended,  to  the 
immaterial  or  unextended  side  of  the  universe  of  being. 


NEEVOUS  AND  MENTAL  FORCES.         217 

When,  therefore,  we  talk  of  incorporating  mind 
with  brain,  we  must  be  held  as  speaking  under  an  im- 
portant reserve  or  qualification.  Asserting  the  union 
in  the  strongest  manner,  we  must  yet  deprive  it  of  the 
almost  invincible  association  of  union  in  place.  An  ex- 
tended organism  is  the  condition  of  our  passing  into  a 
state  where  there  is  no  extension.  A  hximan  being  is 
an  extended  and  material  thing,  attached  to  which  is 
the  power  of  becoming  alive  to  feeling  and  thought,  the 
extreme  remove  from  all  that  is  material ;  a  condition 
of  trance  wherein,  while  it  lasts,  the  material  drops  out 
of  view — so  much  so,  that  we  have  not  the  power  to 
represent  the  two  extremes  as  lying  side  by  side,  as  con- 
tainer and  contained,  or  in  any  other  mode  of  local  con- 
junction. The  condition  of  our  existing  thoroughly  in 
the  one,  is  the  momentary  eclipse  or  extinction  of  the 
other. 

The  only  mode  of  union  that  is  not  contradictory  is 
the  union  of  close  succession  in  time  /  or  of  position  in 
a  continued  thread  of  conscious  life.  We  are  entitled 
to  say  that  the  same  being  is,  by  alternate  fits,  object 
and  subject,  under  extended  and  under  unextended  con- 
sciousness ;  and  that  without  the  extended  consciousness 
the  unextended  would  not  arise.  Without  certain  pe- 
culiar modes  of  the  extended — what  we  call  a  cerebral 
organization,  and  so  on — we  could  not  have  those  times 
of  trance,  our  pleasures,  our  pains,  and  our  ideas,  which 


218  THE  CONSEEVATION  OF  ENERGY. 

at  present  we  undergo  HtMly  and  alternatelj  with  our 
extended  consciousness. 

Having  thus  called  attention  to  the  metaphysical  dif- 
ficulty of  assigning  the  relative  position  of  mind  and 
matter,  I  will  now  state  briefly  what  I  think  the  mode 
of  dealing  with  mind  in  correlation  with  the  other  forces. 
That  there  is  a  definite  equivalence  between  mental 
manifestations  and  physical  forces,  the  same  as  between 
the  pliysical  forces  themselves,  is,  I  thinlc,  conformable 
to  all  the  facts,  although  liable  to  peculiar  difliculties  in 
the  way  of  decisive  proof: 

I.  The  mental  manifestations  are  in  exact  proportion 
to  their  physical  supports. 

If  the  doctrine  of  tlie  thorough-going  connection  of 
mind  and  body  is  good  for  any  thing,  it  must  go  this 
length.  There  must  be  a  numerically-proportioned  rise 
and  fall  of  the  two  together.  I  believe  that  all  tlic  un- 
equivocal facts  bear  out  this  proportion. 

Take  first  the  more  obvious  illustrations.  In  the 
employment  of  external  agents,  as  warmth  and  food,  all 
will  admit  that  the  sensation  rises  exactly  as  the  stimu- 
lant rises,  until  a  certain  point  is  reached,  when  the 
agency  changes  its  character ;  too  great  heat  destroy- 
ing the  tissues,  and  too  much  food  impeding  digestion. 
There  is,  although  we  may  not  have  the  power  to  fix  it, 
a  sensational  equivalent  of  heat,  of  food,  of  exercise,  of 
Bound,  of  light ;  there  is  a  definite  change  of  feeling,  an 


NEEVOUS  AND  MENTAL  FOKCES.        219 

accession  of  pleasure  or  of  pain,  corresponding  to  a  rise 
of  temperature  in  the  air  of  10°,  20°,  or  30°.  And 
so  with  regard  to  every  other  agent  operating  upon 
the  human  sensibility :  there  is,  in  each  set  of  cir- 
cumstances, a  sensational  equivalent  of  alcohol,  of  odors, 
of  music,  of  spectacle. 

It  is  this  definite  relation  between  outward  agents 
and  the  human  feelings  that  renders  it  possible  to  dis- 
cuss human  interests  from  the  objective  side,  the  only 
accessible  side.  "VVe  cannot  read  the  feelings  of  our  fel- 
lows; we  merely  presume  that  like  agents  will  affect 
them  all  in  nearly  the  same  way.  It  is  thus  that  we 
measure  men's  fortunes  and  felicity  by  the  numerical 
amount  of  certain  agents,  as  money,  and  by  the  absence 
or  low  degree  of  certain  other  agents,  the  causes  of  pain 
and  tlie  depressors  of  vitality.  And,  although  the  esti- 
mate is  somewhat  rough,  this  is  not  owing  to  the  indefi- 
niteuess  of  the  sensational  equivalent,  but  to  the  com- 
plications of  the  human  system,  and  chiefly  to  the  nar- 
rowness of  the  line  that  everywhere  divides  the  whole- 
some from  the  unwholesome  degrees  of  all  stimulants. 

Let  us  next  represent  the  equivalence  under  vital  or 
physiological  action.  The  chief  organ  concerned  is  the 
brain  ;  of  which  we  know  that  it  is  a  system  of  myriads 
of  connecting  threads,  ramifying,  uniting,  and  crossing 
at  innumerable  points ;  that  these  threads  are  actuated 
or  made  alive  with  a  current  influence  called  the  nerve 


220        THE  CONSERVATION  OF  ENERGY. 

force  ;  that  this  nerve-force  is  a  member  of  the  group  of 
correlating  forces ;  that  it  is  immediately  derived  from 
the  changes  in  the  blood,  and  in  the  last  resort  from  oxi- 
dation, or  combiistioA,  of  the  materials  of  the  food,  of 
which  combustion  it  is  a  definite  equivalent.  We  know, 
further,  that  there  can  be  no  feeling,  no  volition,  no  in- 
tellect, without  a  proper  supply  of  blood,  containing 
both  oxygen  and  the  material  to  be  oxidized ;  that,  as 
the  blood  is  richer  in  quality  in  regard  to  these  constit- 
uents, and  more  abundant  in  quantity,  the  mental  pro- 
cesses are  more  intense,  more  vivid.  "We  know  also 
that  there  are  means  of  increasing  the  circulation  in  one 
organ,  and  drawing  it  off  from  another,  chiefly  by  call- 
ing the  one  into  greater  exercise,  as  when  we  exert  the 
muscles  or  convey  food  to  the  stomach ;  and  that,  when 
mental  processes  are  more  than  usually  intensified,  the 
blood  is  proportionally  drawn  to  the  brain ;  the  oxidiz- 
ing process  is  there  in  excess,  with  corresponding  defect 
and  detriment  in  other  organs.  In  high  mental  excite- 
ment, digestion  is  stopped ;  muscular  vigor  is  abated 
except  in  the  one  form  of  giving  vent  to  the  feelings, 
thoughts,  and  purposes ;  the  general  nutrition  languish- 
es; and,  if  the  state  were  long  continued  or  oft  re- 
peated, the  physical  powers,  strictly  so  called,  would 
rapidly  deteriorate.  We  know,  on  the  other  extreme, 
that  sleep  is  accompanied  by  reduced  circulation  in  the 
brain  ;  there  is  in  fact  a  reduced  circulation  generally ; 


NERVOUS  AND  MENTAL  FOECES.  221 

wliile  of  that  reduced  amount  more  goes  to  tlie  nutritive 
functions  tban  to  the  cerebral. 

In  listenino;  to  Dr.  Franldand's  lecture  on  "  Muscular 
Power,"  delivered  at  the  Royal  Institution  of  London,  I 
noticed  that,  in  accounting  for  the  various  items  of  ex- 
penditure of  the  food,  he  gave  "mental  work"  as  one 
heading,  but  declined  to  make  an  entry  thereinunder. 
I  can  imagine  two  reasons  for  this  reserve,  the  state- 
ment of  which  will  further  illustrate  the  general  ipo&i- 
tion.  In  the  first  place,  it  might  be  supposed  that  mind 
is  a  phenomenon  so  anomalous,  uncertain,  so  remote 
from  the  chain  of  material  cause  and  efiect,  that  it  is 
not  even  to  be  mentioned  in  that  connection. 

To  which  I  should  say,  that  mind  is  indeed,  as  a 
phenomenon,  widely  different  from  the  physical  forces, 
but,  nevertheless,  rises  and  falls  in  strict  numerical  con- 
comitance with  these :  so  that  it  still  enters,  if  not  di- 
rectly, at  least  indirectly,  into  the  circle  of  the  cor- 
related forces.  Or,  secondly,  the  lecturer  may  have  held 
that,  though  a  definite  amount  of  the  mental  manifesta- 
tions accompanies  a  definite  amount  of  oxidation  in  the 
special  organs  of  mind,  there  is  no  means  of  reducing 
this  to  a  measure,  even  in  an  approximate  way.  To 
this  I  answer,  that  the  thing  is  difficult  but  not  entirely 
impracticable.  There  is  a  possibility  of  giving,  approx- 
imately at  least,  the  amount  of  blood  circulating  in  the 
brain,  in  the  ordinary  waking  state ;  and,  as  during  a 


222  THE  CONSERVATION  OF  ENEEGY. 

period  of  intense  excitement  we  know  tliat  there  is  a 
general  reduction,  almost  to  paralysis,  of  the  collective 
vital  functions,  we  could  not  be  far  mistaken  in  saying 
that,  in  that  case,  perhaps  one-half  or  one-third  of  all 
the  oxidation  of  the  body  was  expended  in  keeping  up 
the  cerebral  fires. 

It  is  a  very  serious  drawback  in  any  department  of 
knowledge,  where  there  are  relations  of  quantity,  to 
be  unable  to  reduce  them  to  numerical  precision.  This 
is  the  case  with  mind  in  a  great  degree,  although  not 
with  it  alone  ;  many  physical  qualities  are  in  the  same 
state  of  unprecise  measurement.  "We  cannot  reduce 
to  numbers  the  statement  of  a  man's  constitutional 
vigor,  so  as  to  say  how  much  he  has  lost  by  fatigue,  by 
disease,  by  age,  or  how  much  he  has  gained  by  a  certain 
healthy  regimen.  Undoubtedly,  however,  it  is  in  mind 
that  the  difficulties  of  attaininor  the  numerical  statement 
are  greatest  if  not  nearly  insuperable.  When  we  say 
that  one  man  is  more  courageous,  more  loving,  more 
irascible  than  another,  we  apply  a  scale  of  degree,  exist- 
ing in  our  own  mind,  but  so  vague  that  we  may  apply 
it  differently  at  different  times,  while  we  can  hardly 
communicate  it  to  others  exactly  as  it  stands  to  our- 
selves. The  consequence  is,  that  a  great  margin  of  al- 
lowance must  always  be  made  in  those  statements  ;  we 
can  never  run  a  close  argument,  or  contend  for  a  nice 
shade  of  distinction.     Between  the  extremes  of  timidity? 


NERVOUS  AND  MENTAL  FOECES.         223 

and  couraore  of  character  the  best  observer  could  not 
entertain  above  seven  or  eight  varieties  of  gradation, 
while  two  different  persons  consulting  together  could 
hardlj  agree  upon  so  minute  a  subdivision  as  that. 
The  phrenologists,  in  their  scale  of  qualities,  had  the 
advantage  of  an  external  indication  of  size,  but  they 
must  have  felt  the  uselessncss  of  graduating  this  beyond 
the  delicacy  of  discriminating  the  subjectiv^e  side  of 
character;  and  their  extreme  scale  included  twenty 
steps  or  interpolations. 

Making  allowance  for  this  inevitable  defect,  I  will 
endeavor  to  present  a  series  of  illustrations  of  the  prin- 
ciple of  correlation  as  applied  to  mind,  in  the  manner 
explained.  I  deal  not  with  mind  directly,  but  with  its 
material  side,  with  whose  activity,  measured  exactly  as 
we  measure  the  other  physical  forces,  true  mental  activ- 
ity has  a  definite  correspondence. 

Let  us  suppose,  then,  a  human  being  with  average 
physical  constitution,  in  respect  of  nutritive  vigor,  and 
fairly  supplied  with  food  and  with  air,  or  oxygen.  The 
result  of  the  oxidation  of  the  food  is  a  definite  total  of 
force,  which  may  be  variously  distributed.  The  demand 
made  by  the  brain,  to  sustain  the  purely  mental  func- 
tions, may  be  below  average,  or  above  average ;  there 
will  be  a  corresponding,  but  inverse,  variation  of  the 
remainder  available  for  the  more  strictly  physical  pro- 


224:  THE  CONSEEVATION  OF  ENEEGY. 

cesses,  as  muscular  po"sverj  digestive  power,  animal  heat, 
and  so  on. 

In  the  first  case  s^apposed,  tlie  case  of  a  small  demand 
for  mental  work  and  excitement,  we  look  for,  and  we 
find,  a  better  jphysiqiie — greater  mnscular  power  and 
endurance,  more  vigor  of  digestion,  rendering  a  coarser 
food  snfiicient  for  nourisliment,  more  resistance  to  ex- 
cesses of  cold  and  heat ;  in  short,  a  constitution  adapted 
to  physical  drudgery  and  physical  hardship. 

Take,  now,  the  other  extreme.  Let  there  be  a  great 
demand  for  mental  work.  The  oxidation  must  now  be 
disproportionately  expended  in  the  brain  ;  less  is  given 
to  the  muscles,  the  stomach,  the  lungs,  the  skin,  and  se- 
creting organs  generally.  There  is  a  reduction  of  the 
possible  muscular  work,  and  of  the  ability  to  subsist  on 
coarser  food,  and  to  endure  hardship.  Experience  con- 
firms this  inference ;  the  common  observation  of  man- 
kind has  recognized  the  fact — although  in  a  vague,  un- 
steady form — that  the  head-worker  is  not  equally  fitted 
to  be  a  hand-worker.  The  master,  mistress,  or  overseer 
has  each  more  delicacy  of  sense,  more  management, 
more  resource,  than  the  manual  operatives,  but  to  these 
belongs  the  superiority  of  muscular  power  and  persist- 
ence. 

There  is  nothing  incompatible  with  the  principle  in 
allowing  the  possibility  of  combining,  under  certain 
favorable  conditions,  both  physical  and  mental  exertion 


NESVOUS  AND  MENTAL  FORCES.        225 

in  considerable  amount.  In  fact,  the  principle  teaches 
us  exactly  liow  the  thing  may  be  done.  Improve  the 
quality  and  increase  the  quantity  of  the  food  ;  increase 
the  supply  of  oxygen  by  healthy  residence  ;  let  the  ha- 
bitual muscular  exertion  be  such  as  to  strengthen  and 
not  impair  the  functions  ;  abate  as  much  as  possible  all 
excesses  and  irregularities,  bodily  and  mental ;  add  the 
enormous  economy  of  an  educated  disposal  of  the  forces ; 
and  you  will  develop  a  higher  being,  a  greater  aggregate 
of  power.  You  will  then  have  more  to  spare  for  all 
kinds  of  expenditure — for  the  physico-mental,  as  well 
as  for  the  strictly  physical.  "What  other  explanation  is 
needed  of  the  military  superiority  of  the  officer  over 
the  common  soldier  ?  of  the  general  efficiency  of  the 
man  nourished,  but  not  enervated,  by  worldly  abun- 
dance ? 

It  may  be  possible,  at  some  future  stage  of  scientific 
inquiry,  to  compute  the  comparative  amount  of  oxida- 
tion in  the  brain  during  severe  mental  labor.  Even 
now,  from  obvious  facts,  we  must  pronounce  it  to  be  a 
very  considerable  fraction  of  the  entire  work  done  in 
the  system.  The  privation  of  the  other  interests  during 
mental  exertion  is  so  apparent,  so  extensive,  that  if  the 
exertion  should  happen  to  be  long  continued,  a  liberal 
atonement  has  to  be  made  in  order  to  stave  off  general 
insolvency.  Mental  excess  counts  as  largely  as  muscu- 
lar excess  in  the  diversion  of  power ;  it  would  be  com* 


226  'i'SE  CONSEEVATION  OF  ENEEGY. 

potent  to  suppose  either  tlie  one  or  tlie  other  redu- 
cing the  remaining  forces  of  the  system  to  one-half  of 
their  proper  amount.  In  both  cases,  the  work  of  resto 
ration  must  be  on  the  same  simple  plan  of  redressing 
the  inequality,  of  allowing  more  than  the  average  flow 
of  blood  to  the  impoverished  organs,  for  a  length  of 
time  corresponding  to  the  period  when  their  nourish- 
ment has  been  too  small.  It  is  in  this  consideration  that 
we  seem  to  have  the  reasonable,  I  may  say  the  arith- 
metical, basis  of  the  constitutional  treatment  of  chronic 
disease.  "VYe  repay  the  debt  to  Nature  by  allowing  the 
weakened  organ  to  be  better  nourished  and  less  taxed, 
according  to  the  degradation  it  has  undergone  by  the 
opposite  line  of  treatment.  In  a  large  class  of  diseases 
we  have  obviously  a  species  of  insolvency,  to  be  dealt 
with  according  to  the  sound  method  of  readjusting  the 
relations  of  expenditure  and  income.  And,  if  such  be 
the  true  theory,  it  seems  to  follow  that  medication  is 
only  an  inferior  adjunct.  Drugs,  even  in  their  happiest 
application,  can  but  guide  and  favor  the  restorative  pro- 
cess ;  just  as  the  stirring  of  a  fire  may  make  it  burn, 
provided  there  be  the  needful  fuel. 

There  is  thus  a  definite,  although  not  numerically- 
statable  relation,  between  the  total  of  the  physico-mental 
forces  and  the  total  of  the  purely  physical  processes. 
The  grand  aggregate  of  the  oxidation  of  the  system  in- 
cludes both  ;  and,  the  more  the  force  taken  up  by  one, 


NEEVOUS  AND  MENTAL  F0ECE8.        227 

tlie  less  is  left  to  the  otlier.  Sucli  is  the  statement  of 
the  correlation  of  mind  to  the  other  forces  of  Nature. 
We  do  not  deal  with  pure  mind — mind  in  the  abstract ; 
we  have  no  experience  of  an  entity  of  that  description. 
We  deal  with  a  compound  or  two-sided  phenomenon — 
mental  on  one  side,  physical  on  the  other ;  there  is  a 
definite  correspondence  in  degree,  although  a  difference 
of  nature,  between  the  two  sides  ;  and  the  physical  side 
is  itself  in  full  correlation  with  the  recognized  physical 
forces  of  the  world. 

II.  There  remains  another  application  of  the  doc- 
trine, perhaps  equally  interesting  to  contemplate,  and 
more  within  my  special  line  of  study.  I  mean  the  cor- 
relation of  the  mental  forces  among  themselves  (still 
viewed  in  the  conjoint  arrangement).  Just  as  we  assign 
limits  to  mind  as  a  whole,  by  a  reference  to  the  grant 
of  physical  expenditure,  in  oxidation,  etc.,  for  the  de- 
partment, so  we  must  assign  limits  to  the  different 
phases  or  modes  of  mental  work — thought,  feeling,  and 
BO  on — according  to  the  share  allotted  to  each  ;  so  that, 
while  the  mind  as  a  whole  may  be  stinted  by  the  de- 
mands of  the  non-mental  functions,  each  separate  mani- 
festation is  bounded  by  the  requirements  of  the  others. 
This  is  an  inevitable  consequence  of  the  general  princi- 
ple, and  equally  receives  the  confirmation  of  experience. 
There  is  the  same  absence  of  numerical  precision  of  es- 
timate ;  our  scale  of  quantity  can  have  but  few  divisions 


228  THE  CONSEEVATION  OF  ENEKGY. 

between  the  highest  and  the  lowest  degrees,  and  these 
not  well  fixed. 

What  is  required  for  this  application  of  the  princi- 
ple is,  to  ascertain  the  comparative  cost,  in  the  physical 
point  of  view,  of  the  different  functions  of  the  mind. 

The  great  divisions  of  the  mind  are — feeling,  will, 
and  thought ;  feeling,  seen  in  our  pleasures  and  pains  ; 
will,  in  our  labors  to  attain  the  one  and  avoid  the  other  ; 
thought,  in  our  sensations,  ideas,  recollections,  reason- 
ings, imaginings,  and  so  on.  ISTow,  the  forces  of  the  mind, 
with  their  physical  supports,  may  be  evenly  or  unevenly 
distributed  over  the  three  functions.  They  may  go  by 
preference  either  to  feeling,  to  action,  or  to  thinking ; 
and,  if  more  is  given  to  one,  less  must  remain  to  the 
others,  the  entire  quantity  being  limited. 

First,  as  to  the  feelings.  Every  throb  of  pleasure  costs 
something  to  the  physical  system  ;  and  two  throbs  cost 
twice  as  much  as  one.  If  we  cannot  fix  a  precise  equiv- 
alent, it  is  not  because  the  relation  is  not  definite,  but 
from  the  difficulties  of  reducing  degrees  of  pleasure  to 
a  recognized  standard.  Of  this,  however,  there  can  be 
no  reasonable  doubt — namely,  that  a  large  amount  of 
pleasure  supposes  a  corresponding  large  expenditure  of 
blood  and  nerve-tissue,  to  the  stinting,  perhaps,  of  the 
active  energies  and  the  intellectual  processes.  It  is  a 
matter  of  practical  moment  to  ascertain  what  pleasures 
coet  least,  for  there  are  thrifty  and  unthrifty  modes  of 


NEEVOUS  AND  MENTAL  FOKCES.         £29 

epending-  our  brain  and  heart's  blood.  Experience 
probably  justifies  ns  in  saying  that  the  narcotic  stimu- 
lants are,  in  general,  a  more  extravagant  expenditure 
than  the  stimulation  of  food,  society,  and  fine  art.  One 
of  the  safest  of  delights,  if  not  very  acute,  is  the  delight 
of  abounding  physical  vigor  ;  for,  from  the  very  suppo- 
sition, the  supply  to  the  brain  is  not  such  as  to  interfere 
with  the  general  interests  of  the  system.  But  the  the- 
ory of  pleasure  is  incomjDlete  without  the  theory  of 
pain. 

As  a  rule,  pain  is  a  more  costly  experience  than 
pleasure,  although  sometimes  economical  as  a  check  to 
the  spendthrift  pleasures.  Pain  is  physically  accom- 
panied by  an  excess  of  blood  in  the. brain,  from  at  least 
two  causes — extreme  intensity  of  nervous  action,  and 
conflicting  currents,  both  being  sources  of  waste.  The 
sleeplessness  of  the  pained  condition  means  that  the  cir- 
culation is  never  allowed  to  subside  from  the  brain  ;  the 
irritation  maintains  energetic  currents,  which  bring  the 
blood  copiously  to  the  parts  aifected. 

There  is  a  possibility  of  excitement,  of  considerable 
amount,  without  either  pleasure  or  pain  ;  the  cost  here 
is  simply  as  the  excitement :  mere  surprises  may  be  of 
this  nature.  Such  excitement  has  no  value,  except  in- 
tellectually ;  it  may  detain  the  thoughts,  and  impress 
the  memory,  but  it  is  not  a  final  end  of  our  being,  as 
pleasure  is ;  and  it  does  not  waste  power  to  the  extent 


230  THE  CONSERVATION   OF  ENEEGI. 

that  pain  does.  The  ideally  best  condition  is  a  moder- 
ate surplus  of  pleasure — a  gentle  glow,  not  rising  into 
brilliancy  or  intensity,  except  at  considerable  intervals 
(say  a  small  portion  of  every  day),  falling  down  frequent- 
ly to  indifference,  but  seldom  sinking  into  pain. 

Attendant  on  strong  feeling,  especially  in  constitu- 
tions young  or  robust,  there  is  usually  a  great  amount 
of  mere  bodily  vehemence,  as  gesticulation,  play  of 
countenance,  of  voice,  and  so  on.  This  counts  as  mus- 
cular work,  and  is  an  addition  to  the  brain-work.  Prop- 
erly speaking,  the  cerebral  currents  discharge  themselves 
in  movements,  and  are  modified  according  to  the  scope 
given  to  those  movements.  Eesistance  to  the  move- 
ments is  liable  to  increase  the  conscious  activity  of  the 
brain,  although  a  continuing  resistance  may  suppress 
the  entire  wave. 

Next  as  to  the  will,  or  our  voluntary  labors  and 
pursuits  for  the  great  ends  of  obtaining  pleasure  and 
warding  off  pain.  This  part  of  our  system  is  a  com- 
pound experience  of  feeling  and  movement ;  the  proi> 
erly  mental  fict  being  included  under  feeling — that  is, 
pleasure  and  pain,  present  or  imagined.  When  our 
voluntary  endeavors  are  successful,  a  distinct  throb  of 
pleasure  is  the  result,  which  counts  among  our  valuable 
enjoyments :  when  they  fail,  a  painful  and  depressing 
state  ensues.  The  more  complicated  operations  of  the 
will,  as  in  adjusting  many  opposite  interests,  bring  in 


r 


NEKVOUS   AND  MENTAL  FOECES.  231 

tlie  element  of  conflict,  which  is  always  painful  and 
wasting.  Two  strong  stimulants  pointing  opposite 
ways,  as  when  a  miser  has  to  pay  a  high  fee  to  the  sur- 
geon that  saves  his  eyesight,  occasion  a  fierce  struggle 
and  severe  draft  upon  the  physical  supports  of  the  feel- 
ings. 

Although  the  processes  of  feeling  all  involve  a  mani- 
fest, and  it  may  be  a  serious,  expenditure  of  physical 
power,  which  of  course  is  lost  to  the  purely  physical 
functions ;  and  although  the  extreme  degrees  of  pleas- 
ure, of  pain,  or  of  neutral  excitement,  must  be  adverse 
to  the  general  vigor ;  yet  the  presumption  is,  that  we 
can  afford  a  ceitain  moderate  share  of  all  these  without 
too  great  inroads  on  the  other  interests.  It  is  the 
thinking  or  intellectual  part  of  us  that  involves  the 
heaviest  item  of  expenditure  in  the  phy  si  co-mental  de- 
partment. Any  thing  like  a  great  or  general  cultiva- 
tion of  the  powers  of  thought,  or  any  occupation  that 
severely  and  continuously  brings  them  into  play,  will 
induce  such  a  preponderance  of  cerebral  activity,  in  ox- 
idation and  in  nerve-currents,  as  to  disturb  the  balance 
of  life,  and  to  require  special  arrangements  for  redeem- 
ing that  disturbance.  This  is  fully  verified  by  all  we 
know  of  the  tendency  of  intellectual  application  to  ex- 
haust the  physical  powers,  and  to  bring  on  early  decay. 

A  careful  analysis  of  the  operations  of  the  intellect 

enables  us  to  distinguish  the  kind  of  exercises  that  in- 
11 


232  T^^^  CONSEKVATION  OF  ENERGY. 

volve  the  greatest  expenditure,  from  the  extent  and 
the  intensity  of  the  cerebral  occupation.  I  can  but 
make  a  rapid  selection  of  leading  points : 

First.  The  mere  exercise  of  the  senses,  in  the  way 
of  attention,  with  a  view  to  watch,  to  discriminate,  to 
identify,  belongs  to  the  intellectual  function,  and  ex- 
hausts the  powers  according  as  it  is  long  continued,  and 
according  to  the  delicacy  of  the  operation ;  the  mean- 
ing of  delicacy  being  that  an  exaggerated  activity  of  the 
organ  is  needed  to  make  the  required  discernment.  To 
be  all  day  on  the  qui  vive  for  some  very  slight  and  bare- 
ly perceptible  indications  to  the  eye  or  the  ear,  as  in 
catching  an  indistinct  speaker,  is  an  exhausting  labor 
of  attention. 

Secondly.  The  work  of  acquisition  is  necessarily  a 
process  of  great  nervous  expenditure.  Unintentional 
imitation  costs  least,  because  there  is  no  forcing  of  re- 
luctant attention.  But  a  course  of  extensive  and  vari- 
ous acquisitions  cannot  be  maintained  without  a  large 
supply  of  blood  to  cement  all  the  multifarious  connec- 
tions of  the  nerve-fibres,  constituting  the  physical  side 
of  acquisition.  An  abated  support  of  other  mental  func- 
tions, as  well  as  of  the  purely  physical  functions,  must 
accompany  a  life  devoted  to  mental  improvement, 
whether  arts,  languages,  sciences,  moral  restraints,  or 
other  culture. 

Of  special  acquisitions,  languages  are  the  most  ap 


NEKVOUS  AND  MENTAL  FORCES.         2oS 

parent!  J  voluminous ;  but  tlie  memory  for  visible  or  pic- 
torial aspects,  if  very  liigb,  as  in  the  painter  and  the 
picturesque  poet,  makes  a  prodigious  demand  upon  the 
plastic  combinations  of  the  brain. 

The  acquisition  of  science  is  severe,  rather  than  multi- 
farious ;  it  glories  in  comprehending  much  in  little,  but 
that  little  is  made  up  of  painful  abstract  elements,  every 
one  of  which,  in  the  last  resort,  must  have  at  its  beck  a 
host  of  explanatory  particulars  :  so  that,  after  all,  the 
burden  lies  in  the  multitude.  If  science  is  easy  to  a  se- 
lect number  of  minds,  it  is  because  there  is  a  large  spon- 
taneous determination  of  force  to  the  cerebral  elements 
that  support  it ;  which  force  is  supplied  by  the  limited 
common  fund,  and  leaves  so  much  the  less  for  other 
uses. 

If  we  advert  to  the  moral  acquisitions  and  habits  in 
a  well-regulated  mind,  we  must  admit  the  need  of  a  large 
expenditure  to  build  up  the  fabric.  The  carefully- 
poised  estimate  of  good  and  evil  for  self,  the  ever-present 
sense  of  the  interests  of  others,  and  the  ready  obedience 
to  all  the  special  ordinances  that  make  up  the  morality 
of  the  time,  however  truly  expressed  in  terms  of  high 
and  abstract  spirituality,  have  their  counterpart  in  the 
physical  organism  ;  they  have  used  up  a  large  and  defi- 
nite amount  of  nutriment,  and,  had  they  been  less 
developed,  there  would  have  been  a  gain  of  power  to 
some  other  department,  mental  or  physical. 


234  THE  CONSERVATION  OF  ENEEGY. 

Refraining  from  furtlier  detail  on  this  head,  I  close 
the  illustration  bj  a  brief  reference  to  one  other  aspect 
of  mental  expenditure,  namely,  the  department  of  intel- 
lectual production,  execution,  or  creativeness,  to  which 
in  the  end  our  acquired  powers  are  ministerial.  Of 
course,  the  greater  the  mere  continuance  or  amount  of 
intellectual  labor  in  business,  speculation,  fine  art,  or 
any  thing  else,  the  greater  the  demand  on  the  jphysique. 
But  amount  is  not  all.  There  are  notorious  differences 
of  severity  or  laboriousness,  which,  when  closely  exam- 
ined, are  summed  up  in  one  comprehensive  statement — 
namely,  the  number,  the  variety,  and  the  conflicting  na- 
ture of  the  conditions  that  have  to  be  fulfilled.  By  this 
we  explain  the  difficulty  of  work,  the  toil  of  invention, 
the  harassment  of  adaptation,  the  worry  of  leadership, 
the  responsibility  of  high  office,  the  severity  of  a  lofty 
ideal,  the  distraction  of  numerous  sympathies,  the  meri- 
toriousness  of  sound  judgment,  the  arduousness  of  any 
great  virtue.  The  physical  facts  underlying  the  mental 
fact  are  a  wide-spread  agitation  of  the  cerebral  currents, 
a  tumultuous  conflict,  a  consumption  of  energy. 

It  is  this  compliance  with  numerous  and  opposing 
conditions  that  obtains  the  most  scanty  justice  in  our 
'appreciation  of  character.  The  unknown  amount  of 
painful  suppression  that  a  cautious  thinker,  a  careful 
writer,  or  an  artist  of  fine  taste,  has  gone  through,  rej)- 
resents  a  great   phy  si  co-mental    expenditure.     The  re- 


NERVOUS  AND  MENTAL  FORCES.         235 

gard  to  evidence  is  a  heavy  drag  on  the  wings  of  specu- 
lative daring.  The  greater  the  number  of  interests  that 
a  political  schemer  can  throw  overboard,  the  easier  his 
work  of  construction.  The  absence  of  restraints — ol 
severe  conditions — in  fine  art,  allows  a  flush  and  ebulli- 
ence, an  opulence  of  production,  that  is  often  called  the 
highest  genius.  The  Shakespearean  profusion  of  images 
would  have  been  reduced  to  one-half,  if  not  less,  by  the 
self-imposed  restraints  of  Pope,  Gray,  or  Tennyson. 
So,  reckless  assertion  is  fuel  to  eloquence.  A  man  of 
ordinary  fairness  of  mind  would  be  no  match  for  the  wit 
and  epigram  of  Swift. 

And  again.  The  incompatibility  of  diverse  attri- 
butes, even  in  minds  of  the  largest  compass  (which  sup- 
poses equally  large  physical  resources),  belongs  to  the 
same  fundamental  law.  A  great  mind  may  be  great  in 
many  things,  because  the  same  kind  of  power  may  have 
numerous  applications.  The  scientific  mind  of  a  high 
order,  is  also  the  practical  mind;  it  is  the  essence  of  rea- 
son in  every  mode  of  its  manifestation — the  true  philos- 
opher in  conduct  as  well  as  in  knowledge.  On  such  a 
mind  also,  a  certain  amount  of  artistic  culture  may  be 
superinduced;  its  powers  of  acquisition  may  be  extended 
so  far.  But  the  spontaneous,  exuberant,  imaginative 
flow,  the  artistic  nature  at  the  core,  never  was,  cannot 
be,  included  in  the  same  individual.  Aristotle  could 
not  be  also  a  tragic  poet ;  nor  Kewton  a  third-rate  por- 


236  THE  CONSEEVATION  OF  ENERGY. 

trait-painter.  The  cost  of  one  of  the  two  modes  of  in- 
tellectual greatness  is  all  that  can  be  borne  by  the  most 
largely-endowed  personality;  any  appearances  to  the 
contrary  are  hollow  and  delusive. 

Other  instances  could  be  given.  Great  activity  and 
great  sensibility  are  extreme  phases,  each  using  a  large 
amount  of  power,  and  therefore  scarcely  to  be  coupled 
in  the  same  system.  The  active,  energetic  man,  loving 
activity  for  its  own  sake,  moving  in  every  direction, 
wants  the  delicate  circumspection  of  another  man  who 
does  not  love  activity  for  its  own  sake,  but  is  energetic 
only  at  the  spur  of  his  special  ends. 

And  once  more.  Great  intellect  as  a  whole  is  not 
readily  united  with  a  large  emotional  nature.  The  in- 
compatibility is  best  seen  by  inquiring  whether  men  of 
overflowing  sociability  are  deep  and  original  thinkers, 
great  discoverers,  accurate  inquirers,  great  organizers  in 
affairs ;  or  whether  their  greatness  is  not  limited  to  the 
spheres  where  feeling  performs  a  part — poetry,  elo- 
quence, and  social  ascendency. 


TUB    END. 


INDEX 


Absorbed  heat  changed  into  chemi- 
cal separation,  114. 

into  actual  visible  energy,  105. 

into  light  and  beat,  117. 
Acquisition,  232. 
Actinic  rays,  129. 

Action  and  reaction  equal  and  op- 
posite, 8. 
Affinity,  chemical,  53. 
Air  and  water  in  motion,  ll'?. 
Albuminoids,  177,  183. 
Amber,  61. 
Ampere,  75. 
Amyloids,  177,  183. 
Ancients,  their  ideas  not   prolific, 

135. 
Andrews,  141. 
Animal  heat,  207. 
Animals,  how  they  live,  188. 
Animals  and  inanimate  machines, 

165. 
Aristotle  on  a  medium,  134. 

on  mind  and  body,  207. 
Atmospheric  circulation,  109. 
Atomic  forces  and  heat,  58. 
Atomic  or  chemical  separation,  80. 
Atoms  and  molecules,  51. 
Attention,  232, 
Attraction,  molecular,  52. 

mutual,  of  atoms,  64. 

and  repulsion  of  magnets,  75. 

of  electric  currents,  75, 

Bacon,  133,  137. 
Battery  of  firove,  70. 
Budding,  180. 

Caloric,  38. 
Carnivora,  189. 
Chemical  affinity,  53. 

and  electrical  attraction,  64. 

and  heat,  58. 


Chemical    combination    producing 

heat,  119. 
Chemical  instability,  156. 
Chemical  separation  converted  into 
electrical  separation,  122. 

into  electricity  in  motion,  123. 
Chlorophyll,  177. 
Chrysalis,  187. 

Circulation  of  the  atmosphere,  109. 
Clausius,  141, 
Cohesion,  force  of,  51. 
Cold  apparently  produced   by  the 

electric  current,  1 26. 
Conduction  of  electricity,  61. 
Conservation,  laws  of,  82, 

theory  of,  140. 
Crossbow  and  watch-spring,  25. 
Current,  the  electric,  69. 

and  magnetism,  72. 

heating  effect  of,  73. 

chemical  effect  of,  74. 
Currents,   electric,   attraction    and 
repulsion  of,  74. 

induction  of,  75. 

Dalton,  133. 

Davy,  Sir  Humphrey,  38,  137.  J 
Democritus  on  atoms,  133. 
Descartes,  136. 
Diastase,  184. 
Disease-germs,  3. 
Dissipation  of  energy,  141. 
Dissociation,  116. 

Egg,  development  of  the,  186. 
Electric  current,  69. 

and  magnetism,  72. 

heatmg  effect  of,  73. 

chemical  effect  of,  74. 

induction,  65. 
Electrical  attraction  and  chemical 
affinity,  64. 


238 


INDEX. 


Electrical  separation,  81. 
when  produced,  64. 
transmuted  into  visible  motion, 

124. 
into  electric  current,  124. 
Electro-magnetism,  72. 
Elastic  forces,  50. 
Electricity,  60. 

vitreous  and  resinous,  63. 
negative  and  positive,  63. 
theory  of,  63. 
in  motion,  81. 

transmuted    into   visible   mo- 
tion, 124. 
into  heat,  125. 

into  chemical  separation,  127. 
Encke's  comet,  96. 
Energies,  list  of,  78-82. 

natural,  and  their  sources,  143. 
Energy,  meaning  of,  1-22. 

of  bodies  in   motion  propor- 
tional   to    their   weight   or 
mass,  14. 
proportional  to  the  square  of 

the  velocity,  19. 
of  visible  motion,  its  transmu- 
tation, 87. 
visible,   transformed   into   ab- 
sorbed heat,  88. 
dissipation  of,  141. 
transmutations  of,  27. 
varies  as  the  square  of  the  ve- 
locity, 15. 
of  motion,  24. 

transformed  into  electrical  sep- 
aration, 98. 
of  position,  a  sort  of  capital, 
26. 
Equilibrium,  154. 
Etiolation,  180. 

Fermentation,  183. 

Food,  145. 

Force,  vital,  whence  derived,  171. 

physical,  194. 

clicmical,  194. 

of  chemical  affinity,  53. 

of  cohesion,  51. 


Force,  mechanical  or  molar,  205. 

molecular,  205. 
Friction,  35. 

Heat,  absorbed,  changed  into  chemi- 
cal separation,  114. 
into  electrical  separation,  115. 
into  electricity  in  motion,  116. 

Heat-units  of  different  substances, 
119. 

Heat-motion,  80. 

Heat-engines,  their  essential  condi- 
tions, 107. 

Helmhollz,  141. 

Heraclitus  on  energy,  133. 

Herbivora,  191. 

Heterogeneity  essential  in  electrical 
development,  64. 

Huyghens,  137. 

Hydraulic  press,  32. 

Inclined  plane,  28. 
Incubation,  186. 
Individuals,  our  ignorance  of,  1. 
Induction,  electric,  65. 

of  electric  currents,  75. 
Instabihty,  mechanical,  155.   • 

chemicnl,  156. 
Intellectual  labor,  234. 

Joule,  137,  140,  141. 
Joule's  experiments  on  work  and 
heat,  44. 

Kilogrammetre,  16. 

Larva,  187. 

Latent  heat,  57. 

Laws  of  conservation,  82. 

Life  depends  on  the  sun,  165. 

Light,  a  perpetual,  impossible,  149. 

Lime,  carbonate,  easily  decomposed, 

58. 
List  of  energies,  78-82. 

Machines,  their  true  function,  33. 
animated  and  inanimate,  157. 


INDEX. 


28& 


Magnets,  attachraeut  and  repulsion 

of,  75. 
Maxwell,  141.] 
Mayer,  140. 

Mechanical    energy    changed    into 
heat,  23. 
equivalent  of  heat,  43. 
force,  205. 
instability,  155. 
Mental  forces,  mutual  correlations 

of,  227-236. 
Mind,    its   correlations   to    natural 
forces,  218-227. 
and  body,  207,  211. 
Molar  force,  205. 
Molecular  attraction  and  heat,  55. 

separation,  80. 
Molecules,  ultimate,  of  matter,  5. 
their  motions,  7. 
and  atoms,  51. 
Motion   changed   into    an   electric 

current,  99. 
Muscular  power,  207. 

Xarcotic  stimulants,  229. 
Negative  and  positive  electricity,  63. 
Nerve  power,  207. 
Newton,  136,  137. 
Non-couductors  of  electricity,  61. 

Percussion,  36. 

Perpetual  motion,  139. 

Physical  ibree,  194. 

Plants  growing  at  night,  181 

Positive  and  negative  electrici+v,  63. 

Protoplasm,  177. 

Pulleys,  their  function,  30. 

Radiant  energy,  81. 

converted  into  absorbed  heat, 

123. 
promoting     chemical    separa- 
tion, 123. 
Rankine,  141. 

Resinous  and  vitreous  electricity, 63. 
Rotation  of  earth  retarded,  95. 
Rumford,  39,  137. 


Silver  oxide  readily  decomposed,  58. 
Solar  rays,  decomposition  by,  59. 
Sulphur,  146. 
Sun — a  source  of  high-temperaturfi 

heat,  148. 
Sun's  heat,  origin  of,  150. 

spots,    auroras,    and    cyclones 

correlated,  98. 

Tait,  141. 

Temperature  of  dissociation,  115. 

Thermo-electricity,  116. 

Thermopile,  117. 

Thomas  Aquinas,  209. 

Thomson,  William  and  James,  140. 

Tides,  146. 

Tissues,  decay  of,  164. 

Universe,  its  probable  fate,  152 
Units  of  heat  and  work,  46. 

Vegetation,  176. 

Velocity  and   energy,  relation  be- 
tween, 16. 
Virtual  velocities,  34. 

principle  of,  its  history,  137. 
Vital  force,  whence  derived,  171. 
Vitality,  194. 
Vitreous  and   resinous   electricity, 

63. 
Voltaic  current,  69. 

and  magnetism,  72. 

heating  eSect  of,  73. 

chemical  effect  of,  74. 

Water  at  high  level,  24. 

Watt,  138. 

Wild's    electro-magnetic   machine, 

103. 
Will,  194. 

Work,  definition  of,  J5. 
unit  of,  15. 

rise   of    true   conceptions  r& 
garding,  138. 

Yeast-plant,  185. 


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